Possibly the source of any confusion here is when the encryption and when the compression takes place? Maybe some more details about how you are using xz and encryption would help.
As far as I can tell, xz doesn’t do anything with signatures or encryption, but it does perform checksums like you stated, which is very cool and I’m glad you shared this.
Edit: I am re-reading your post above. You are compressing with xz, then encrypting, got it. So yes, if any part of the payload is tampered with, then it would be detected by the decryption, depending on the algorithm, or by the decompression because of the checksums like you said. Sorry for the confusion! You’ve got it all straight lol.