Very slow start
That's the thing...there is no 'default expectation'. If the GM has a strong preference, they should make that clear in their RTJ information, but not everyone does. When I've written backgrounds for my characters, they've always been third person, and I've never had anyone knock me for it. Almost all of the games I've been in have had people posting in third-person/past tense, a few people prefer present tense, and every game I've been in where the GM had a definite preference one way or the other, they made it known.
I can see why a GM would ask for an in-character background, as it gives you a chance to express the voice of the character as opposed to just cataloging the events of the character's life. And, for what it's worth, I agree that the GM should have stated they were expecting the background to be written in-character.
But that is entirely a matter of GM preference. It may be that particular DM is looking for players who default to writing in-character when not told otherwise...might be his way of pre-screening for players whose style he most prefers. That's his prerogative. And if he did it that way, and it bugs you, I'd say that's a good sign to bow out of the game now. If the GM's style is already rubbing you the wrong way, I consider that a strong indicator that the GM and player are not going to jive well with each other and that just seems to make playing a chore, in my experience.
I will say, by way of caveat, that if he hasn't already selected against you, airing your grievance in a relatively public venue like this so soon isn't likely to help your case. A lot of GMs are unlikely to gladly accept a player who is demonstrating a lack of patience...anyone who's been on here long enough has experienced at least one occasion where Real Life reached out and messed up a GM's regularly-scheduled posts, and if being a few days behind creates extreme frustration, and they know about it in advance, there's a good chance they'll say, "I'm just gonna spare both of us the potential headaches and go with someone who's more patient..."