That's a different situation. Boarding time is often up to half an hour before departure. If you're at the gate but sitting (rather than queuing anxiously or even, as some people do, trying your best to board first for some reason) then they're not going to deny you boarding simply for being one of the last people to board the plane. If there's a gap between the queue ending before you arrive, then you're more likely to be denied boarding.
Boarding first or near the first is much better when you end up with a seat near the back, since it means you can just walk all the way through a mostly empty plane, put your stuff in the empty overhead compartments, and get seated without having to make your way through a narrow corridor filled with other people fiddling with their stuff. If your seat is near the door, then the opposite of this advice holds: be one of the last to enter the plane, and you won't have to get jostled around by everyone else behind you walking by. I think if everyone queued up in this order (furthest from door enter first, closest to door enter last) it would make boarding a lot more efficient.
