Hiya!
Two things I'll bring up...
(1) PHB != MM, and MM != PHB. In non-programmer speak, they are two different books, and have some
significantly different purposes and "rules". Look at, say, the "Evil Mage" for an example; the Evil Mage simply
does not follow the same rules as the PHB lays out for wizards. Why would anyone in their right mind assume that everything else found in the MM wouldn't likewise follow those same "...it's a monster and doesn't have to play by the same rules as the PC's"?
(2) "Multiattack" is not quite the same as "Multi-Attack", nor is it the same as "Multiple Attacks". At least not any more than "Great Weapon Fighter" is the same as "Greatweapon Fighter".
Under "Multiattack" description in the beginning of the MM, it says it is "multiple attacks", and this
does seem to indicate that it's the 'same' as in the PHB. Looking at the monsters description of "Multiattack", however, it specifically states what constitutes the monsters "choice" of choosing Multiattack...just like a PC fighter can choose Push as one of his Attacks...but only one. Why can the PC do it and not a monster? Because "push" is an option for a PC to be uses as any ONE of his attacks. If a monster was to use a "Push" attack option, that's what he's doing as his action; in other words, he isn't using his Multiattack option.
So, how I'd read it, is much like Mr.Mearls has; monsters "multiattack" is not the same as a PC with multiple attacks. That said, I'd have no problem what so ever completely ignoring his way of running his game....as I run my game the way I want. Meaning that my rulings supersede him every time.
^_^
Paul L. Ming