This is a tricky problem. I also use Text Chat for online role playing but right now I can't think a definite solution or suggestion.
What do YOU do? Have you not experienced a problem there?
In our last session the PCs have arrived at a baron's manor and it was something like this:
GM: It is getting late and the sun is setting, but despite the late hour the baron will see you in his study.
Player 1: It's late, so he won't see us right now. We need to find a place to stay for the night.
Player 2: We should look for a room somewhere.
GM: HELLO! I said the baron will see you now!
:-) Everyone's typing at the same time and whoever hits enter first gets his line out.
Yes, I've experienced the same problem. The indication of "The GM is typing..." isn't enough for many people . . . and in many cases all of them should stop typing or refraining from pushing Enter while the GM is typing .
What I do?
Currently I manage this "text box chaos" by having very few players, even just the GM and a single player, but a such approach isn't the idea of a virtual tabletop.
Honestly, I'm waiting for the Roll20 Team's implementation of more Text Chat Box features, that have been requested in other threads. For instance, the use of a "stop" signal/icon inside the Text Chat Box (or something analogous to that) would have avoided the misunderstanding in your example, because by seeing that, the players just let the GM to finish what he's saying. Otherwise he must try to type all, more or less quickly, in a single paragraph before hitting Enter in order to not suffering inopportune interruptions in this or that "chunk" of his narration.
All this would be fine if more people, instead just wanting to see Roll20 copying MapTool's features for maps, would want the same thing but also regarding Fantasy Grounds and its features for Text Chat, that are the most developed so far in the world of virtual tabletops.
WebCam and voice is good, but for online role playing, I always prefer text. If I'm using a WebCam at all, I reserve WebCam and voice for out of character (and out of game) chat, pre-game clarifications and such things.