No. It doesn't.
The Oxford English Dictionary traces singular they back to 1375, where it appears in the medieval romance William and the Werewolf
The New Oxford Dictionary of English (1998) not only accepts singular they, they also use the form in their definitions.
And the New Oxford American Dictionary (Third Edition, 2010), calls singular they ‘generally accepted’ with indefinites, and ‘now common but less widely accepted’ with definite nouns, especially in formal contexts.
Please, learn English.
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?
You have the air of that fundamentalist branch that forked off in central north america to worship a prescriptive Elements of (Our) Style by Stunk & Whine.
P2: The person in the green coat?
P1: Yes, that one.
P2: I don't know them. Do you think Joe knows who they are?
Language evolves so paper dictionaries from the beginning of the 21st century will not be entirely accurate but given the way some publishers of online dictionaries have taken to using their products to push language revolution instead of following language evolution those old paper books are more accurate guides to how people use it than their ideologically-driven on-line counterparts.
It's been common place all my life and that's going on six decades now.
'Merkin English, however, likely has prescriptive rails and a lingering shrinking horror stemming from the inappropriate touch of Webster.
> those old paper books are more accurate guides to how people use it than their ideologically-driven on-line counterparts.
Exactly - see the full OED (Oxford English Dictionary) entry on the long historic use of 'they' over seven centuries and the many ways in which it has been (and continuous to be) used contemporaneously both as singular and plural.
However, for certain using plurals for individuals of unknown gender is hardly rare, there are hundreds of examples in literature a quick Google away.
It has become (somewhat) popular in the context particularly of discussions on trans rights and 'misgendering' to take the position that the singular they is somehow ungrammatical because people don't want to use it to refer to people whose gender they believe is not unknown.
But it seems to me as though that argument is actually not related to the current discussion.
Here we are simply discussing whether 'they' can be used to refer to a person whose gender is unknown to the speaker at the time, for which there is ample historical, literary evidence that this is commonplace in English, as well as the anecdotal evidence from sibling comments.