How would Bill Gates copy source code from a 36-bit minicomputer with 32 kilowords (no byte addressing) of memory and a time-sharing operating system to a 8 bit microprocessor with a completely different instruction set and 4 kilobytes of memory and no operating system, just bare metal? Even if he and Allen had had the source code for BASIC-10, which you haven't provided evidence of, it would be closer to a reimplementation than a port.

And DEC was in Massachusetts, Bill Gates went to high school in Washington. That would be one hell of a road trip to dig into DEC's trash.

I think it was C-Cubed's trash, but it was DEC's IP. See: https://www.theregister.com/2000/06/29/bill_gates_roots/