This is a more synergetic move given that Marvel Studios and Sony have partnered up for Spider-Man’s MCU exploits, and it makes even more sense to tack the trailer onto Avengers: Endgame given that while that movie was the last installment of the Infinity Saga, Spider-Man: Far From Home is the last movie of Phase 3.
It’s not often that you see a movie trailer tagged onto a movie after it’s finished playing.
However, it does remind me how back when The Amazing Spider-Man 2 came out, a teaser scene from X-Men: Days of Future Past was shown due to a deal between director Marc Webb and 20th Century Fox.