That. Is. And. Ever. Was. Simply. Wrong!
I really don’t know where that „fact“ is coming from. That’s not how software development works and would be totally pointless.
Just look at the compile-dates of the kernel of a random beta build: it was compiled some DAYS before that build went live.
At some time, they branch off a build from the main development tree and define it as a beta seed. And that ist usually a few days before that build is released. Then this build is compiled - quality checked - recompiled if needed (and that are the small letter at the end of the build) and tested again. But that usually does not take more than a week.
So even internally, they are not „weeks ahead“ or have the completed builds of upcoming betas lying around and just let time pass.