I’m not HCM, but my understanding is that as long as you don’t make it a machine gun, put a stock on it (making an SBR) or remove, change or obliterate the serial number you can cut up a receiver however you want to. You can’t “manufacture” a pistol that is already on the books as a pistol. I’m thinking that person is getting the 80% reciever rules confused with what you’re doing.
ETA: I’ll re-read the relevant laws this evening to double check.
Double edit: 1911 smiths weld up frame rails so the gun is temporarily unusable and re-mill them all the time.