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.
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).
Fascists were not exactly fans of anti-fascists, so while I'd definitely agree that Antifa isn't currently repressed, this has not always been the case.
https://en.wikipedia.org/wiki/Amersfoort_concentration_camp