No it's not
And there is no good reason for that other than Apple chose to lock it down.
They absolutely could have made user accessible high spec NVMe (at least for additional storage for certain) and chose not to.
This is the nickel and dime, upsell, dongle, "services" company now.
Every single micro decision they make now is to squeeze another nickel.
Nothing is done for the benefit of users anymore
They are using a proprietary SSD interconnect, so standard NVME drives would not work. But I don’t think there is any technical reason against using some sort of proprietary SSD socket.
Funnily enough, I think one of the most valid reasons not to use a replaceable SSD is that people will probably complain that it’s not a standard M.2 connector and future legislations (like an extreme version of right of repair) might force Apple to use standard SSDs. By using a soldered-in SSD they can just say that it’s an integral part of the system.