This is just a random Facebook page.

It’s also their colors and groups sometimes coöpt imagery of their adversaries, so it’s not unprecedented for a repressed group to take a symbol back for themselves (not saying antifa is repressed, but they act on behalf of other groups).

It's wrong to generalize from one page linked to a specific city in the Netherlands; during WW2 there was a small concentration camp at Amersfoort, a suburb of Utrecht about 10km from the city center. That likely resonated with contemporary anti-fascists in Utrecht, but the red triangle is absolutely not in general use as an Antifa symbol.

https://en.wikipedia.org/wiki/Amersfoort_concentration_camp