That line of intervention holds for correcting a general social offence, however in this instance the competing goal of welcoming the first time attendee must be considered.

On the grand scale of things, perhaps Submitter C values correcting potential gender bias in favour of gender neutrality/awareness more than promoting welcoming behaviour; this interpretation conflicts with Submitter C's explicitly stated intent for intervening and is not a charitable reading. I will only address Submitter C's explicit objection of the implications, meant or not, of Guy's inquiry of the first time attendee.

According to their submission, Submitter C wished to prioritise welcoming the first time attendee over chastising Guy; I will assume chastising Guy was a sub-goal considering the content of the exchange.

Wedrifid points out that having a public outburst introduces negativity and political signalling to the situation's social dynamic; without firm knowledge of the first time attendee's thoughts or emotions regarding Guy's inquiry (an affect does not a credible signal make), a positive intervention would have the greater probability of welcoming the attendee.

GUY: "So, do you actually read Less Wrong, or did someone drag you here?"

SUBMITTER C (grinning/smiling): "That's not a very welcoming phrasing, now is it?"

Guy's social ineptitude, but innocent intent (actual intent becomes irrelevant) are signalled to the first time attendee; Guy is mildly nudged to think on their phrasing without feeling (too) victimised, and most importantly the first time attendee is welcomed with a signal of group care for kindness. More chastising can be given to Guy by Submitter C later and in privacy, where Guy will not have frets about status fettering an update.