You are supposed to use pic-as, but is no direct substitute for MPASM.
Your assembly source WILL requiring modifying to work with pic-as.
Well, from what I found out just lately, this doesn't even seem like a viable solution at all.
From what I understand, pic-as isn't even a full blown assembler and it was only meant to handle mix assembly code embedded in C code, and not 100% assembly stand alone.
Not knowing any of this, when I was trying to get this set up, I couldn't make this work, and as I found out a few details, I ended up downgrading mplabx to v5.3 to regain the use of mpasmx.
I won't be doing any C coding at all, only 100% pure assembly, so pic-as doesn't look well suited at all for that.