Never heard of this app, but sounds interesting! Gotta love the gig economy...

I think that was specifically related to a separate functionality, "The same app discusses apps that connect passengers with generous travel allowances to folks who need stuff shipped."
But even if one is a low-level elite with a 1-bag allowance, I'm sure many times the traveler is only doing carry-on, so there still exists the opportunity to "sell" that allowance to someone else via an app. Do any elite programs/levels allow 3 bags? I think most first/business class tickets do, I suppose that's a "loophole" they could close, especially if you figure the logistics of handling one third-party bag are probably not worth the money you'd make, but maybe 2-3 are.

I would also think there could be security implications to checking a bag that belongs to someone else, although perhaps with all bags being screened, it's less of an issue these days.