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.
[edit: On second thought I'll not walk into this minefield of a thread quite this nonchalantly with an "actually".]
The symbol does exist in a certain context of Antifa/ARA imagery, but if you use it in full context it just makes the Trump team look even worse, surprised the media hasn't picked up on this angle yet.