Sorta. One potion until out of combat limitation is correct. if thats what potion sickness is used for.
but, the invisible debuff no icon, I've never seen or experienced. Its just , usable if its off cooldown, unusable if its on cooldown.
The picture in the link, shows a potion off cooldown, but being unable to be used because of potion sickness. I think that must have been in wotlk beta, because I've never seen anything about it on official. plus. The comments were from 2.4.3 using a screenshot from wotlk beta.
I don't think potion sickness exists on official when they are out of combat. The picture shows him in combat having no cooldown on potion but unable to use it. It should be usable.
Its just that part that's confusing me.