Why would my Xfinity gateway fail to allow just airbnb?

My computers and mobile devices' apps and browsers can't load airbnb.com IF they're on my own Xfinity wifi. I get "network error" or the infamous "429 Too Many Requests" message. This is true of the airbnb app as well as webpages.

But I CAN get airbnb.com fine if:
> I'm using iphone's ATT data and have wifi disabled
> I'm using my laptop / imac on my iphone's hotspot
> I'm using someone ELSE's Xfinity wifi / gateway.

I can access all other webpages through my gateway, it's just airbnb that's an issue.

This may have started a couple days ago when my Xfinity service was mysteriously cancelled. Today they reinstated it and re-activated the router. So it seems to be Xfinity, right? But why would only one website (so far) be affected?