AFAIK the other Pixel devices, going back to the 8, do use MTE in the stock OS. Some MTE features are only enabled when "Advanced Protection" is turned on in the OS settings, but the stock OS on those devices does make use of MTE. In the Android documentation, MTE is explicitly called out as one of the protection features enabled in the Advanced Protection mode.

So, I don't think it's accurate to say they don't use it yet. They were using it, and then they stopped. I think that's one of the reasons the linked GOS thread says they're concerned that "MTE may actually be broken due to CPU errata".