I’m not sure I understand. The advice was:
But as far as I know, the activity wasn’t incorrect, it was just very old and (I would guess) irrelevant to my current fitness. I went back and deleted the activities, and my calculated TP is still -10W. Can someone explain this to me like I’m a total newbie, and used to just doing what I’m told? 
Update: Just for the heck of it, I went into my Profile, and clicked “Recalculate Progression” after deleting the old activities from a few years ago. Got the warning that I should make sure my “Fitness Signature Is Set Accurately Before Proceeding” so I went to the Threshold Power box and bumped it from 265 to 275 like it was before. Then recalculated everything and my TP came out to 263. I guess the algorithm’s punishing me for trying to game the system. lol