IDK, they said "a terrible way to make tea", I'm assuming there's some sorta tea lore that I'm unaware of
I think the theory is that "stale" water lacks sufficient oxygen for tea brewing to fully brew or something.
I'm not a big tea drinker, but I am definitely not a super fan of water that's been sitting out a day. There's not been time for stuff to grow in it yet, but it still tastes gross.