Curious what is pointing to this?
Well, simply, the fact that more than six months have passed after M1 announcement and no other variant has turned up. And besides, prosumer hardware would benefit from faster clocks and new features (like aforementioned SVE), so the more time passes, the less likely it is that it will be based on Firestorm cores.
Apple wants their chips to be the best and they have something to prove. M1 is excellent entry-level, but the prosumer version has to be better in every regard, including single-threaded performance. I doubt that Firestorm can be clocked higher than it's current 3.2Ghz limit. Hence a new core that supports higher performance (either via wider backend or higher clocks).
My skepticism is that an M2X (higher-end chip) would be created after the M2, which would be created after A15. So they’d likely announce those chips first. Also the 16” MBP hasn’t been updated in a long time. Rumors pointed to WWDC but possible chip shortages created delays, but that would likely mean an M1X (whatever it’s called) instead of a second gen chip.
I think the prosumer platform will look rather differently. First of all, I don't think it will be delivered as an SoC, but rather as a system on a package, with CPU/GPU being on different chips. Second, as I wrote before, prosumer hardware will likely have some new features that current Firestorm-based M1 lacks. And finally, just because Apple historically went from a smaller chip to a larger one, does not mean that they need to continue doing so. M1 is a direct application of Apple's mobile tech, as it's basically an A14X (it uses the same packaging technology as the A12Z for example) with more I/O. While you can continue doing the same thing for prosumer chips, you will run into issues with yields and configurability (a one-size-fits-all M1 works fine, but will the same strategy work for pro-level machines?). I believe that future Apple Silicon models will be more diversified and also differ more from their mobile variants. That is, it would make more sense for Apple to develop multiple systems in parallel, using the same architectural base, rather then continuing the line of A14->M1->M1X (whatever).