You are free to state your opinion, of course. But in this case you attempted a diagnosis while not being qualified for that. Not sure if 1st ammendment protects that.
Fear is not classified as a disease and therefore cannot be considered a diagnosis (which is defined as: Noun, The identification of the nature of an illness or other problem by examination of the symptoms.) in the context that you are trying to cast it. Your assertion that Mary was diagnosing anything is incorrect.
She ventured an opinion (which is defined as: Noun, A view or judgment formed about something, not necessarily based on fact or knowledge.), in this case based on something that Marc himself wrote. Publishing on the internet for public consumption implicitly invites opinionated responses unless comments are explicitly forbidden or otherwise limited by the author.