Behlendorf's comment in that thread seems to be talking about linux integration. My point was this is an older issue, going back to the Sun days. See for instance this thread in where McVoy complains about the same issue! https://www.tuhs.org/pipermail/tuhs/2021-February/023013.htm...
That seems more like it's complaining about it not being the actual page cache, not it not being counted as "cache", which is a larger set in at least Linux than just the page cache itself.
But sure, it's certainly an older issue, and given that the ABD rework happened, I wouldn't put anything past being "feasible" if the benefits were great enough.
(Look at the O_DIRECT zvol rework stuff that's pending (I believe not merged) for how a more cut-through memory model could be done, though that has all the tradeoffs you might expect of skipping the abstractions ZFS uses to minimize the ability of applications to poke holes in the abstraction model and violate consistency, I believe...)
