I think they were saying that they could save space by converting their existing jpg files to avif or jpgXL, not converting to a 70% quality jpg. JpgXL can do this losslessly so there’s no drawback there, but converting to avif would be a lossy to lossy transcode.
EDIT: I completely missed OP’s last paragraph, which does say they are considering converting their existing jpg files into 70% jpgs.