If I remember correctly the claim was from a Chinese citizen who pointed this out and when Signal didn't treat this as a major security issue (which I felt was understandable given that her concern was mostly along the lines of "you can't say that Signal is secure given that people install other software on their phone that could be a keylogger so you should have a disclaimer") she decided to continue to escalate on social media until she was, IMO, harassing Moxie for his "lack of engagement". I didn't come away particularly impressed by that exchange.