Well, even if the idea isn’t total BS it doesn’t explain the “need” to spread so wide.
I took a look at this last year on the London Underground. Most men do sit with their legs apart, (surprise!) but its a minority that do so with a spread wider than the seat space. This occurred across all the age groups but apart perhaps from the very large men, there seemed no reason why they might not sit with a more reasonable spread. it also occurred regardless of who was sitting next to them.
A small number of women also showed this “behaviour” but it was much less than in men.
Overall having observed this, I would suggest it is more an expression of “entitlement” ( thus a form of rudeness to others) than a specifically sexist activity. It has to be considered however that it will be experienced differently by those it happens to, which is perhaps why women feel it is more oppressive to them; not that that matters- as it is still rude.
Essentially, a small number of ignorant men have managed to create a distorted view of male behaviour. Most spreaders only spread within the allocated seat space, but will create the impression, because of the ruder minority, that the behaviour is more widespread. :-)