I only found out recently that I have a bunch of allergies that were responsible for my rapidly worsening health (asthma and ulcerative colitis, for what it's worth, hospitalised several times for both in the past few years).
So I'd just like to say a couple of things:
Firstly, it's not for you to question the legitimacy of other people's food allergies or intolerances.
I've had a tonne of this, with people thinking I'm just being pale and interesting, and no amount of explanation of how much better I am since I cut out these allergens (medication need reduced by 90%, symptoms likewise, and no hospitalizations) is good enough for them, so I've stopped bothering to explain it. People who really care about me understand, the ones who just want to cast aspersions on the liegitamcy of what I'm doing or bitch about the inconvenience can do one, frankly.
And it's naive to think that it's either epi-pen time or a fake. There are a bunch of us who won't go into anaphylactic shock if we eat a certain thing, but it might make us suffer crippling cramps, bleeding, asthma attacks, or any number of other deeply unpleasant reactions.
And on the inconvenience thing – I'm very much aware that it's my problem, not anyone else's, and that at catered events there may very well be nothing I can eat. So I eat before I go, take a snack, or stay hungry. Mostly I'll just pick the bits I can eat and go with that. I think most people with allergies do similarly.
Thing is though, you asked them, so they told you. So you can't really now bitch about it or accuse them of being fakes because you don't like the answer.
You can accommodate their allergies or not, to whatever degree, and then graciously just let them know what's going to be on offer so they can plan accordingly. And they probably will - because they're just trying to manage their conditions, not trying make your life difficult for shits and giggles (if you'll pardon the pun).
If you want practical advice (and I suspect you don't):
You can cover off a lot of allergies by avoiding gluten in main courses (ie. pasta), offering a vegan option, and avoiding nuts. Most caterers can handle that without blinking. Hopefully then most of your guests will be able to enjoy something of what's on offer.
And it's perfectly fine when you let them know your plans to ask if anyone is at risk from cross- contamination, as you're not sure if the catering environment will be 100% cross-contamination proof - that way you can head off any life threatening situations. Although most of the people I know with these kinds of allergies are very good about letting people know unprompted anyway.
It's just not cool to imply that your guests are faking, just because you're poorly informed.