Currently available flash is obviously unusable. HBF is not that.

The reason HBF is (about to be) a thing is that flash manufacturers realized that if you heavily optimize flash for read throughput and energy, as opposed to density, you can match DRAM on throughput and get to within 2x on energy, at the cost of half your density. That would make the density still ~50 times better than DRAM, built on a cheap mass-produced process. All manufacturers are chasing this hard right now, with first samples to arrive later this year.

You are correct that it would absolutely not be used for any mutable data, only weights in inference. This is both because there is insufficient endurance (expected to be ~hundreds of drive writes total), but also because it will be very slow to write compared to the read speed. A single HBF stack is expected to provide 1.6TB/s reads, and single-digit GB/s writes. That's why I wrote the last sentence of my post that you replied to.