I don't remember the original message you are mentioning, and I don't know if it was from the same person, I was just referring to that specific message and of course I agree with your point, and I said it myself in the comment you quoted, touch on Mac isn't making much difference. The main reason why Macs don't sell as much as PCs (and, by the way, iPads no longer outsell Macs) is price and, to some extent, "inertia" (people stay with what they know, unless they have a very compelling reason to change). Enabling the Pencil on Macs (with or without touch), would probably have a bigger impact than allowing touch, but sales would still be limited to a niche of artists and other people who want a Mac as their main device and need/want to draw, annotate etc.
It wouldn't make much difference in terms of market share. Even Apple Silicon, which is a much bigger deal than touch/pencil, hasn't moved the market share needle much. Only price could (and even then it would take time to win the inertia), but Apple is not and will never be a high volume - low margin company...