The funny bit about this is that the principles that broke SIDH were in the literature --- they owe to a late-1990's theorem† by Ernst Kani, a mathematician in Ontario. We spoke to Steven Galbraith about this (I wouldn't know who Kani was if I hadn't read Galbraith, just to be clear) and he'd even talked to Kani long before any of this came out. But Kani isn't a cryptographer and apparently isn't even especially interested in cryptography, so the dots didn't get connected until much later.

† That's the "25-year-old theorem" from the article.