Yes, I do know that. I was speaking inexactly.
Except that those rules are not universal. Specific trumps general, and we already have multiple instances of this very rule being overridden by a specific metamagic feat: Substitute Energy is just one of the metamagic feats which explicitly does not require a higher-level slot.To get the benefits of either feat however, you need to either prepare that spell in a higher slot or expend a higher slot (in case of spontaneous casters). You can find those rules in the description of the feat type.