they are not casting spells though, they take the Magic action with an item and replace the Magic action with the Ready action
No they don't When you you ready the Magic action you take the Magic action as a reaction.
Now if you want to get technical, RAW you could Ready the Magic action for after the Cleric casts heal on you.
I take the Ready action to use my Mace of Terror after the Cleric Heals me (use the Ready action on your turn)
Cleric casts Heal you, meaning you are no longer befuddled and the trigger happens
You use your Mace of Terror (take the Magic action as a Reaction)
Technically RAW this is allowed.
What is not allowed is to Ready a spell after the Cleric Heals you, because to do that you need to cast a spell on your turn, which would be before you were healed and you can not cast spells while Befuddled. I can't think of any way to bypass this and enable you to Ready a spell while under Befuddlement, even accounting for what happens off turn.