Yeah, the Alpe. I think people are entering their weights lower.
There's lots of reasons results on zwift can be inaccurate or "unfair" between different riders. As you say they can put a lower than real weight in for themselves, or it could be down to the type of trainer they're using, some of which can be pretty inaccurate.
Not knowing either of you it's impossible for anyone else to say whether you're right or not, but 54 minutes for Alpe Du Zwift isn't that extraordinary on an accurate trainer for a reasonably - but far from elite - rider. We have a Strava group of Cyclechat riders and the top 9 times are under 54 minutes or under (for the 7.59 mile Strava segment called "Alpe Du Zwift (Zwift Insider Verified)"). Most of us in that top 10 are not spring chickens any more either
If you do races on Zwift, are you signed up for Zwiftpower? If not I'd recommend it, as it's a results service for Zwift that does a lot to try and verify fair results - obviously it's not perfect, but it's as good as it gets and improving all the time.
Ultimately, unless you have a group of people you know well and know are using similar and accurate equipment and not "weight doping" it's pointless worrying about other peoples results, but better to concentrate on improving your own.