I think the linked Rootkovska tweet (or more specifically, her own thread/replies to it) actually gracefully acknowledges that Xen is not magically free of attack surfaces.

She's also written some fairly long-form articles about Xen security and the (sometimes significant) room for improvement, including, for example, this Black Hat talk (from 2008!) about breaking Xen: https://invisiblethingslab.com/resources/bh08/part3.pdf

I don't disagree with the rest of your comment :) I just have the impression that any discussion about Qubes+Xen is less "dismissal" and more that supporting a variety of hypervisors is "a small matter of programming" (where "small" is used to mean "absolutely not small").