And those BASIC interpreters were extremely slow. Clearly there are many interpreters and compilers for the 6502 and almost every other CPU architecture ever designed. Nobody said it's impossible to program the 6502 in high level languages, the architecture is just not designed to be a compiler/interpreter target and it is basically impossible to map many high level language constructs to efficient 6502 code.
There is an important difference: BASIC is interpreted, not compiled.
You are erroneously conflating high level and compiled. Or you are not reading what you yourself actually wrote.
I imagine that 'programming the 6502' here refers to somehow creating code that the CPU executes directly, and so interpreted languages are automatically excluded.