no, it really is a true 64 bit CPU. it's just that that fact is useless and it usually runs in 32 bit mode because games don't need that.

Yes. Although people like to point out that on the N64's CPU the external data bus is restricted to 32 bits, that's irrelevant in practice. The real limitation is the RDRAM's data bus, which is only 9 bits wide (of which the CPU uses 8 bits). The problem is that the rest of the system simply cannot match the overspecced CPU.

I wonder what the maximum addressable memory of the N64 is?

Of course, even 32 bit are massively more than they actually need for the paltry amount of memory they actually get, even if you map ROM and various devices into the same virtual address space.