I have the world's most physically fit human being for a middle son.
He uses clips on his geared training bike because he wants to ride in his running shoes.
When I ride with him I notice he reaches down and tightens his straps so that he can't pull out; and, when he comes to a situation where he mignt need to pull out, he reaches down and loosens his straps.
I doubt whether any fixie riders would do that because it seems too hard to do, if a rider can't coast for a second while tightening or loosening the straps.
I assume, then, that most fixie riders who use clips adjust their straps and then leave them at a tightness that lets them slip in and pull out and yet lets them use the whole circle of the spin, or at least enough of it.
Perhaps, though, different riders do different things with their clips and straps.
For example, in my early geared bike riding days, I used rather substantial clips without any straps, and this let me use more of the circle, but nowhere near as much as I use now riding clipless.
Now that I ride fixed, I think sometimes I might like to get a set of pedals with clips and ride with them just for the learning experience.