That top answer is editorializing but their own analysis indicates that in Europe the upside red triangle is incredibly common in resistance movement imagery as well as having precedented use in US Antifa branches.
As far as symbols go, it's fairly good. Simple is better than complex. A red triangle has no difficult lines, curves or fine detail. You can spray paint it, print it, draw it... and it will look pretty much the same every time.
The three flag symbol isn't nearly as easy to make the same everywhere every time without some official stencil or something.
The three flag symbol isn't nearly as easy to make the same everywhere every time without some official stencil or something.