The bacteria on the surface of the meat will hit room temperature in a few minutes, long before the entire piece is thawed all the way through. So it gets an hour head start.

No, it will not. But it depends on the shape of the meat. So if you are not monitoring the surface temperature, go with cold water or the fridge.

By that logic no one could ever safely eat steak tartare, which obviously you can without any issue. There will be bacteria growing in the surface, but unless someone fucked up somewhere along the way, they won't be harmful.

> If you defrost it from frozen to cold-but-defrosted, shouldn't you be ok, even if the surrounding air is room temperature

Yes, that's my point. If you defrost in the fridge it's ok. If you defrost at room temperature then it's dangerous.

It's not extremely dangerous. Many people do it every day and don't die. But it's better not to.

No, what I meant was, if you defrost it at room temperature, it will not instantly warm up to room temperature - it will slowly warm up. You're not forced to leave it on the counter until it reaches room temperature, you can choose to use it before that point (even though it is surrounded by room-temperature air). It's possible to have a less-than-room-temperature fish inside a room that is in room temperature.

The issue is the timing. If you defrost in the fridge, it will eventually reach ambient temperature, but that's still safe. If you defrost on the counter, it will briefly be cold-but-defrosted, but rapidly progress to room-temperature-petri-dish.