If you quicken a real spell, your real action can be a cantrip
If you quicken a cantrip, your real action can be a cantrip
I agree the rule should have been: You can only cast one spell of 1st level or higher per turn.
Another quirk with the existing rule:
Wizard A casts Fireball
Wizard B casts Counterspell as a reaction
Wizard A casts Counterspell on Wizard B's Counterspell
Wizard A's Fireball goes off
Wizard A just cast two level-3 spells in one turn. He cast a reaction while casting another spell. All legal.