The vague accusation is that because "onion routing"[1] has roots in the military, it must have a backdoor that we haven't uncovered in decades. If the person had posted this Wikipedia link with the info you mentioned, for example, I wouldn't have thought it unsubstantial per the guidelines (even if the claim/accusation itself is unsubstantiated by the evidence, that's a difference of opinion and not a guidelines thing).

[1] Not the cryptography, not even the code implementation, but just the general concept: having a message packed in several layers of encryption such that intermediate routers don't know the contents. https://en.wikipedia.org/wiki/Onion_routing