No, there's two restrictions on Multiattack. Why am I posting this for the third time?
"A creature that can make multiple attacks on its turn has the Multiattack ability"
That's it. Extra Attack gets the same text.
"Beginning at 5th leveI, you can attack twice, instead of once, whenever you take the Attack action on your turn"
RAW and RAI, it's pretty clear - the multiple attacks only function on one turn. You say, "It's counter intuitive" when what you really mean to say is, "It's a poor choice to make for your turn". Counter intuitive is, "His familiar can aid on it's own turn, but I have to spend my own turn if I want to command my goat to do the same thing." Is it a poor decision to ready an action? Well when you're readying an action, you typically can't do anything offensive anyways. So the next best thing is to ready an action so you can do something. It's better than nothing.
Even if you try to argue that Multiattack is different, then what does that mean for Druids who specialize in shapeshifting, or arcane casters who use Polymorph? Why are fighters, the masters of martial combat, unable to unleash a flurry of attacks on someone else's turn, but something with 1 Int and a 0 or below modifier to Dex able to act quickly against something they can't even see at the time of readying to strike?!
Adam can run the game how he wants to, but RAW and RAI, it 100%, unambiguously says the opposite.