It's not weird, people been doing that for a long time. I think it's dumb though. People should do them like you do after you hit failure.
I used to do negatives all the time. Very important in tendon strength. I usually only did them on bench and preacher curl. We had those Keiser air machines which are great for negatives. I did the preachers on those.
how is your leg doing?
Do you know if that's all he did? Cuz he might have just finished up several "normal" sets...or was working it into his routine some way you did not take note of.