> it's margin with 100% collateral
Except the collateral hasn't settled yet, so you can't guarantee what could happen.
> It's like if you handed your broker a check and said you wanted to make buy a stock for the value of the check.
There are tons of things that could happen here. The check could bounce. Your bank could go under. The check could be fake. You could have stolen the check from someone else, and when they see it clear, they'll call their bank and the funds will be withdrawn.
> Would the trade clear before the check? Sure. Is that margin?
Yes, it is. The broker may choose to loan you the money short term with the assumption that the check will clear, but they don't have to. They could just refuse to make the trade until the funds clear.
If they did choose to buy the stock for you, they would be accepting that risk. If your money doesn't clear, they will be stuck with the stock.
If this was a share of VTI, with low volatility, they'd likely be fine with it. Even if the check didn't clear, they could just re-sell the stock. It's super unlikely to drastically check in value in the 2 days before the check clears.
If this was a share of GME today, they are buying it for you for $350. There is a change that your check doesn't clear. There is a chance that tomorrow GME is worth $50. If both of those things happen, your broker would have paid $350 for a stock that they didn't want, and can only sell for $50, and be stuck holding a $300 loss.
Ie, they have loaned you money (even if it's short term and with no interest), but if the account doesn't actually get funded, or a previous trade doesn't actually settle[0], they are left holding the bag, the same as if it was "traditional" margin.
[0] This is unlikely, but it could happen for a variety of reasons. I believe this would correlate with much larger issues with the state of the financial system, and in that case would just start compounding issues (because if your trade doesn't settle, then chances are a whole bunch of trades don't settle, and your broker will be left holding the bag for all of them)