It's possible that when switching to PHP7 opcache was enabled by default, whereas it may not have been beforehand. Most likely opcache is responsible for the autoloading improvement.

That said, a faster parser and intepreter would do autoloading faster as well, so it may not be opcache at all and just by virtue that the autoloaded classes are interpreted faster.

I can't imagine that any of those performance gains were related to autoloading.