It apparently used to say it on Valve’s official FAQ for the steam deck, but it’s since been rewritten to only mention the bios method. I found a 4 year old reddit post where they directly quoted the older version of the page.

I also confirmed on my Steam Deck that it works, I was able to get the double blue led flash (which confirms you’ve entered battery storage mode on the OLED) by plugging the deck in, turning it off, holding vol+ and QAM for 10 seconds, and then unplugging the Deck (while still holding the buttons)