"loquitur" is what grammarians have called a deponent verb. It is passive in form (hence the ending) but active in meaning.
That is all you really need to know, but as a bonus: In recent years, cognitive linguists have challenged the idea of "active in meaning" because in these deponent verbs there is almost always some level of "subject affectedness," that is, the actor of the action is also undergoing the action in some way.
If Marcus eats a cookie, it's the cookie that's being eaten up. Or if Marcus sees a book, it's the book that's being seen.
But if Marcus speaks, then it's... uh... well, Marcus that speaks.
(You could argue that Marcus speaks words, and that it's the words that are being spoken. But words are not the focus of "Marcus speaks" -- Marcus is the focus.)
I think that's RyanKaufman's point.
"I am being born" gives that feeling of an action out of your control. Middle English "It thinketh me" (It thinks (to) me - Dative) gives that same feeling of lack of volition. This may be the wrong way to think of Marcus loquitur, but I see it as "Marcus is delivered of speech" or "Marcus is speeched"]. It helps me to get my head around it, anyway.
The difference between doing an action, and undergoing an action (or the action being done to you), is at stake here.
If you think it's not a meaningful difference, then you buy the new-fangled argument. If you think it is a meaningful difference, then you don't. It's one of those "you do you" things.
Personally, i think there is a meaningful difference - so that efforts to confuse or erase the difference is a waste of time and misleading.
"Marcus speaks" has an implied object: the words that he chooses to speak. The full edition would be "Marcus speaks words", or perhaps "Marcus speaks his words." It's just so obvious that it would be stupid for people to keep saying it. So people drop the object..
But the object is still implied. Speaking is something that Marcus does, and his words are what is spoken. Speaking is not something done to Marcus - until we specify a different subject, "Stephanus speaks".