cc65 and ca65 are used mainly for kernel, shell and others binaries. Any assembler can be used, it just needs to add the Orix binary header at the beginning of the binary file.
It's easier to use ca65 because orix sdk provide ca65 macro and .cfg to build a binary when it's coded in assembly only.
Some libs are also build under ca65, and it's easier to link with ca65 if your code is written with ca65.
If you code with cc65 or ca65, you need to use -ttelestrat switch to build Orix binary.