So... what's the "milestone"?

The world’s leading EV maker is preparing to introduce new battery chemistries to stay a step ahead of the competition. After a series of breakthroughs, BYD is expected to begin producing solid-state EV batteries as soon as next year.

Yeah but, what's the milestone?

"Preparing the begin deliveries next year" is not a milestone, it's a plan.

BYD has been researching solid-state batteries since 2013. In 2023, it reached a milestone, demonstrating that mass production was feasible with various cell systems and materials.

Old milestone, I guess?

As others have said, it isn't clear what the 'milestone' is, but:

BYD is now expected to begin producing solid-state batteries in 2027 in limited batches
...
The first two years will serve as a demo phase with limited production. Toward the end of the decade, the company expects to begin mass production as the new batteries are adopted at scale.