Disclaimer: I am not claiming this is a 100% certainty. It is entirely possible Tasfro may surprise us, but given their history it's pretty doubtful.
tl;dr probably even if it's not 100% because tasfro is a small company with shitty programmers.
I'd wager that the possibility of getting another leak like that is extremely high. As for why...it's not nessecairly a mess-up, but it's extremely likely for a number of reasons.
First of all, one has to understand why data mining is even a thing in the first place. Finding dummied out assets, hints of sequels, and even infamous cases like Sun and Moon's demo leaking all of the new mons - all of these things happen because coding is an absolute mess. One would think that it's as simple a matter as just removing the files in question, but then something seemingly unrelated breaks, or the game starts glitching in unexpected ways. This is because even with extremely good coding practices, programming is less like a straight road from A to B, and more like a Jenga tower. You never know what piece you remove is going to cause the whole thing to fall down.
This is, of course, a very simplified explanation of a very complicated topic, but the short version of it is that it's often preferred to simply build upon existing code, rather than trying to modify or remove existing code, and simply hide assets tied in with said code away from the player.
So what does this have to do with a demo of a game? Well, almost all demos (with a few notable exceptions, such as the Stanley Parable's demo) are based off a version of the already existing game. Preferably one later in the game's development cycle for the sake of stability and matching the full experience as much as possible. As a result, a bunch of stuff gets in the code - even for examples like the Sun and Moon Special Demo, where while technically being a unique experience from the game proper was completely based off of the existing code base and assets - and nintendo erred on the side of caution and playability, resulting in some assets making it through.
Nintendo is, by the way, a multi-million dollar company with a history of fairly stable games. Twilight Frontier, meanwhile, is a small company with a history of ...questionable programming quality at best. As a result, it's hardly surprising that they were unable to fully scrub any connections to the full cast in HM or ULiL - as they were presumably set in stone after a certain point. I have no idea and research into the matter didn't reveal if leaks had happened in hisou and prior games - but at this point people have gotten really good at data mining, so it's a whole different ballgame these days to begin with.
Of course, one may raise the question of "Why did the ULiL demo have Hanako listed instead of Sumireko?" The fact that Mokou and Shinmyoumaru were also in that datamine suggests that it wasn't because of an attempt to hide only Sumireko - surely, if the intent was to obscure info until launch, they'd go out of their way to hide the other two as well? Instead, this raises the interesting possibility (and this is only based on speculation from the evidence, unless we hear from ZUN himself we have no way of confirming this) that at some point a character named Hanako was intended to be the final boss of ULiL. This makes sense - Hanako is the name of a fairly big urban legend, and surely it would have made sense to tie in the Urban Legends into the culprit of the plot? I'm only making a wild stab in the dark here, but I'm assuming that ZUN changed his mind and shifted the plot's end focus away from the urban legends and implemented Sumireko instead as a hook for LoLK - presumably out of a desire to start a new meta plot, much like how MoF was the catalyst for SA, UFO, TD, SoPM, and HM. Combined with how Kokoro's entry in the HM demo was simply "boss," and it probably means that both HM and ULiL's demos were based off of considerably earlier builds than the final release - which, if the trend continues, means that we'll probably have to take any non-familiar name with a grain of salt (if we even get one and it's not just boss again). Not because Tasfro's attempting to hide anything, but because the demo will possibly have started being made from a fork that was early on enough that they hadn't fully implemented the final boss yet.
As a side note I fully expect Sagume to be in AoCF. She's gotten a number of cameos recently, she's a stage 5 boss (historically speaking they tend to find themselves in major or playable roles), she was name dropped in Reisen's campaign, featured in the tease for Alt Facts, and seems to have been established as a connection between Lunar Society and Gensokyo, which I'm speculating due to various plot developments and what I assume are bids at foreshadowing will continue to be at the forefront of future events.