You wrote a piece of code while employed. The rights went automatically to the employer, because work for hire.
The employer releases the code under a permissive license on their Github. To everyone.
You don't see any exceptions in the text of the license. The license says that as long as the license and copyright notices are preserved, any rando in the world can do whatever they please with that code. "Any rando" includes but is not limited to: their competitors, the maniac next block who's out to get them, the police, ICE, Hamas terrorists, maybe even their own mothers-in-law. But they don't care, they released it. Period.
Yet somehow you (and a good half of the commenters) imply that there's maybe some unwritten clause that specifically denies the license grant to you, the freaking author of said code, specifically, even though it's not there in the license text.
The mere existence of such thought, and the number of people who subscribe to it, is sickening me to no end. I loathe their filthy minds.