We don't know exactly why BG3 doesn't have a more 5e-like reaction system. That quote implies that Larian internally implemented and tested it, but decided it wouldn't work because [some reason]. Theories for this reason are on previous posts ITT and don't really need restating.
Unfortunately, we as the EA players/testers can't judge whether Larian's implementation of a functioning 5e reaction system is actually tedious or not because we've never been given a chance to test it. We can base our opinions off of other games' implementations, or what we think Larian will do, but that's not quite the same.
Larian has indicated that there is a reaction system overhaul in the works. So who knows what we'll see and when/if we'll see it in EA.
