These are not common, this was the only one I saw, and several divers familiar with the red sea said they are very rare. It is definitely not E. quadricolor. If you saw a lot of them, you saw something else. Is it definitely H. Magnifica? Hard for me to be certain, but looked more like that than any other anemone I've seen. It is a host anemone, so it is likely a document host species, or a completely undocumented one. It is not an obscure described species that no one on this board has heard of.
There is a lot of footage of "anemone city" at Daedalus. Here is 1 example:
https://www.youtube.com/watch?v=SV5q9ya8dhM&ab_channel=SuzannaM/y2013
The Daedalus anemones looked identical except for the color. They are larger than 30cm, but no where near 100cm. They are colonial, and people who dived "anemone city" over the years told me it been getting bigger. They are out in the open getting blown back and forth with very strong current.
I've never seen tentacles shaped like this on a BTA. These look like fat pipes with rounded ends and the same diameter in all parts of every tentacle. BTAs usually either have the 'bulb' ends or are somewhat tapered. I've seen some long thin BTA tentacles that mostly the same diameter everywhere, but never 1/10th this thickness and usually longer, at least in relation to the anemone. I have an H. magnifica in my tank, and the shape is somewhat similar, at least more so than anything else.
If you look at the foot, it is a different color than the tentacles, with a sharp live dividing the colors. I don't think I've ever seen this with a BTA. Multicolored BTA's tend to have gradients and varying colors throughout, not sharp divides with everything else being exactly the same color.