I suspect OP is trying to fight a subconscious bias that the person in question is male, which is why there is inconsistency.
Many of us may also have made a similar assumption–I know I did, and I have certainly made edits to comments in the past where I have unconsciously assumed the gender of someone with no evidence and later noticed. However, we have no idea what gender the user is. English does not have a clear, unambiguous way of saying a third person of an unknown gender, but 'they' is definitely part of common vernacular.
I agree that in general, it's probably better to be consistent internally within a sentence, but the rest of your argument is pretty badly flawed.
With regards to resorting to French to try to argue, 'any online translator' will likely translate 'they walk' to 'ils marchent'. But it's equally possible that it could be 'elles marchent', 'on marche', or with enough context 'il/elle marche' regardless of the gender of the actual person (e.g. 'La personne s'approche dans la nuit. Elle marche avec des pas longs.').
There are also plenty of cases where we refer to singular nouns with plural verbs (e.g. 'the police are on their way').
Prescriptivism in English is an overall unrewarding topic, unlikely to make you any friends, and rarely helps to forward whatever point it is you're trying to make.
I'm not sure that I understand exactly what your point is anyway, how do you propose we refer to someone of unknown gender?
