You know actually, How long does ACTIVE Development take for a Fighter you think
I'm not including the Planning that goes into the character's moveset, art, music, stage, etc, I'm talking about the time where they are actively modeling, animating, programming, balancing the character
Anyway Terry might have not had Gameplay finished to be revealed, but I'm gonna assume they didn't just make Terry in 2-3 months. Lots of planning goes beforehand, and they also have to get permission for everything related to Terry anyway, and that takes awhile as well.
I do think it is weird that Undertale doesn't have spirits, But I'm pretty sure that they had a baseline idea of how the FP2 Roster was gonna look already by the time they announced it, and was just finalizing the details of the actual characters themselves
Consider that Toby is only 1 Man, and he seems like a lax enough guy, It's possible that in 1-2 months, An actual Undertale Fighter could have been decided for the FP2 roster After the reaction to Sans mii costumes, and could explain why we have 6 characters instead of 5. Undertale would be the Extra addition
But I also think there already had to been ideas for Undertale fighter in the first place anyway, since alot does go into these fighters, in terms of planning, to make them to the quality of Smash standard. I'm sure that between Toby and Sakurai, there was discussion of other characters from Undertale besides Sans, but how detailed those discussions were depends
I will say that if we STILL don't get a 2010's Tournament for Smash, than My eyebrows will go up. Pyra/Mythra originated in the 2010's, just like Min Min, and just like Steve, if you start with Minecraft's Java release (which the game does). the tournament should come soon now. If it doesn't think yeah I could maybe see more into this idea of Undertale being decided to get a Full fighter