Well, exactly. These placards are very visible in press photos with poor resolution, even when I'm reading newspaper articles on a tiny phone screen.
But people actually there didn't see them? Not even when they were posing for selfies, after having carefully positioned themselves to capture the crowd in the background?
I can see those signs in the background of Ross Greer's selfie, and most importantly, I can see Kirsten Oswald in front of the signs.
If you had gone up to Ross and Patrick just after that selfie and said, "Ross, Patrick, I heard Stewart McDonald, Kirsten Oswald, Kaukab Stewart and Alison Thewliss were here. Can you point them out to me?" how likely it is that they would have had no idea their political colleagues were attending? I'd say zero.
Now ask yourself whether they would have genuinely had no idea where the four were, even though McDonald, Oswald, Stewart and Thewliss were standing near the statue while holding a # X For Independence" banner? The journalists for the National found the group easily enough and filmed them making a pretty speech about standing with trans siblings.
Greer and Harvie (and the other one) would have known full well where Kirsten Oswald et al were. How odd they never glanced over to see how things were going with the press, and never spotted those signs above her head, eh?