Adverse possession isn’t that simple, and laws regarding it vary by state. In this case, it appears to be Washington state, which requires a number of things that indicate an uphill battle for anyone trying.
Among other requirements, it needs to be uninterrupted (occasional activity doesn’t count), exclusive (the true owner doesn’t use it) for ten entire years, notorious (impossible to miss if you ever are on the property, we’re talking anywhere from fencing it off to building an entire house on it) and hostile (without permission).
So in reality, if I already owned this, avoiding adverse possession on this property is as easy as visiting it once every 5-8 years and telling them to quit the area if they’re trying to elbow their way in (which resets the 10 year clock).
So yeah, not as much a free land grab as one might think.
As a potential purchaser, the greatest concern is if it’s already been more than 10 years. In my mind this is exactly the kind of situation adverse possession is for.
Even if adverse possession doesn’t apply, trying to actually evict those home owners from the land is going to be a nightmare. The validity of the lot itself may even come into question. Generally I would expect the laws governing the creation of lots to try to prevent useless lots like this.
Honestly I don’t understand how lots like this even get listed. I looked at one in my city that was a little piece of a corner between two homes. It’s far too small to build on and you probably couldn’t even fence it off legally. Literally the only thing you could do with it is try to coerce the adjacent homeowner who’s been using it to buy it from you, but that’s just evil, and who wants to be evil?