The Satmar Rebbe, as we know, referred to the Shalosh Shvuos like a De’oraysa of the strongest caliber. His answer to why the Rambam doesn’t mention it, IIRC, is that the Issur of Shalosh Shvuos is a form of Kefira in the Geula. Therefore, the Rambam didn’t mention this particular Gemara that outlines an Onesh once he mentioned Moshiach and the Emuna in a Geula.
Might I also posit that the Satmar Rebbe saw that having a community in Eretz Yisroel under Jewish rule, if viewed positively by his sect, would be the end of the “Shtetel” that he had in Europe, as well as was trying to create (and created) in America (with the insular communities of Williamsburg & Kiryas Yoel). Therefore, in order to forward his vision of a separate community, not only from non-Jews but also from non-Satmars (or better put, those not willing to live like Satmars), he had to make the Jewish Yishuv a non-starter as an issue, and so made opposition to it a cornerstone of Satmarism.
Of course, this is just speculation, but to create as strong of an issur that Satmar did on the state (I feel) has to have more to it than Halacha.
(P.S. I don’t believe this is disrespectful to the Satmar Rov, rather the opposite, as it shows the depth of his vision).