He seems to think that the 'as difficult to prepare and cast' line in Heighten means that it, unlike other metamagics, is ineligible for use if it raises the effective level of the spell past the slots you can normally cast. For example, his interpretation means a Cleric can't DMM: Heighten a spell beyond what they could otherwise cast, or that a Wizard 5/Incantatrix 10 could not cast a level 8 spell Heightened to level 9, despite Improved Metamagic.