Because the manfacturers made the firmware defaults downclock the CPU if it was on battery power.
Also, is it just me or does this article have an "AI generated" feel to it? The arguments it makes are somewhat in the "uncanny valley" between truth and fiction, and the phrasing of some of the sentences seems very unusual, especially this last one:
Stop charging your laptop past 100% today, so you can free yourself from being plugged in tomorrow.
Stuff like this also raises some red flags:
It’s when hen[sic] there are too many lithium ions on one side that the battery gets stressed.