I'm presuming that your sim isn't named Plate? XD

Nope. We just have Marky Sharky and Setra Aura... Which means the game did in fact attribute skill gain to the plate that was being washed....

I've played this game on and off for 17 years... I've seen the rediculous stuck faces... I've seen multiple instances of the same sim arriving at the same lot including one time the sim I was actively playing went to a community lot only for one of himself to be already there playing chess... I've seen bubbles get stuck to a sim's hand, I've seen a sim's head get turned completely backwards... But I have never, ever seen an inanimate object gain a skill point... That's a new one.

Congratulations on Plate, becoming the first object to gain a skill in your hood then