Sam, it does sound very week. However, what if it’s not Halacha per se but something meant to be followed, as are countless other Medrashim? The Rambam does mention that Gemara in his letter. He says to follow the advice of Shlomo Hamelech. In other words, it’s like a so-to-speak Shvua. It is a warning that Shlomo Hamalech is giving with Ruach Hakodesh.
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.
I don’t think the problems began and ended with the Shalosh Shvuos. The Communists and Zionist groups pulled very large numbers of our youth away from Torah and Yiddishkeit. Most European non-Frum Jews left the fold during that period. The Rabbanim witnessed this first-hand. There was basically not a household that was not affected by the new Ruach. This is obviously the driving force behind all the Lamdus.