The screen itself, in most newer enclosures anyway, has nothing to do with the structure. Looking at the picture, it appears as though they used splines on the front to back runs and then, on the ends. If the screen was placed properly, as you insert the spline, it “tightens” the screen. If you were to try and add some splines, it either wouldn’t fit in the grooves or would tear the screen.
The enclosures themselves are strong enough, that, and perhaps John can add to this as he has probably done it this way, you can walk on the beams to install the splines. I saw a couple of guys that rescreened an entire enclosure in an hour! They just rolled out the screen, one guy did one side, the other guy did the other side and they just went right across the screen.
They must have worked in the circus!
On the other hand, when I replaced all of my screens after Wilma, I used some 2 x 12 x 10s so I didn’t end up in my pool…or concrete deck!