This claim is factually incorrect. You ARE locked from progressing the MSQ and experiencing "locked" content until you clear the required group content and get to that point in the MSQ.
There's no way for someone to join you in a dungeon until they've unlocked it via the MSQ. A level 50 character who hasn't progressed the MSQ can't just go to Heavensward content even though they're the appropriate level to start it.
Just because this is how FFXIV does things doesn't mean it's not locking you out from that content.