Let me stop you right there. The Meta Quest 3 is a mixed reality headset as well.
I feel we are getting too caught up in the terminology and trying to classify these devices into neat categories, rather than evaluate what each can do on their own merit.
Let's say I agree with you that both technically count as "mixed reality" headsets, whatever that means. How well does each then fare at a particular task? For example, we have seen people cooking with the Vision Pro headset on, as well as using it in public and even interacting with it in a crowded subway. This is made possible because the sub-12 millisecond passthrough of the vision pro allows users to react to things they see in the camera feed almost as quickly as they are seeing it in real time.
Meanwhile, feedback on the Quest 3's passthrough is that it can get pretty laggy and grainy at times, which may be fine with you are simply using it indoors in a controlled environment, but potentially disastrous if you are attempting to interact with the world around you in public. Apple clearly intends for the vision pro to be reliable enough to be used pretty much anywhere, and it shows in the effort they have made in this aspect, to the point of creating their own custom silicon just for this.
Then there is the ecosystem (like the vision pro automatically connecting to Macs to serve as an external display), and that a cheaper headset invariably means having to compromise in certain areas like passthrough and eye-tracking at a time when the emphasis on them has never been greater.
I feel that Zuckerberg's "Ballmer moment" was when he claimed the vision pro would be too isolating because it didn't involve the metaverse. We now see Facebook adopt a less dismissive tone about how they want to be the android version of the Vision Pro, which I guess is a weird flex, given how android devices tend to have lower revenue per user, as well as lower satisfaction and loyalty rates compared to iOS.
Personally, I have this suspicion that if Meta tries to be the android corollary to the vision pro and fails, they would have inadvertently helped position the Vision Pro alongside the iPad and Apple Watch in terms of not having any genuine, meaningful competition.