The "up to" is quite vague, though, right? I found the
original press release and bizarrely, under footnote 1 (which follows a paragraph about the MBA) it says this was according to tests done with a pre-production MacBook Pro.
Similarly,
on this page on the website, claims are based on "pre-production MacBook Air systems with Apple M1 chip and 8-core GPU, configured with 8GB of RAM and
512GB SSD."
I can't see why any of that would stop Apple from changing the SSD setup in the 256GB model. A MBA M1 configured with 512GB would still be "up to" twice as fast.
But yeah, full disclosure, I don't have legal training and it was just a thought that anyone buying an M1 today based on YouTube videos saying the M1 MBA has faster storage speeds might be disappointed.