If only it wasn’t wrong… 
Well, I don’t think it’s a good comparison.
It’s like saying “Everything you see is light; an apple is something; an orange is also something. So, we can see an apple and an orange. Therefore, an orange and an apple are the same thing”.
I mean, everything being drawn on screen (think about non GUI operating systems, too) doesn’t enforce the fact that a TextArea is based on a canvas.
It is based on a GrafPort, who is the base object of eveything.
Apple and Oranges compare is not to be done.
What I wanted to say that is there only a graphic mode in nowadays computer. Things used in the old green / ambre computers do not exists anymore. A character on screen is just an artwork, not a matrix that have a value.
Probably, but I still fail to see how this means a Xojo TextArea would be based on a Xojo canvas… 
OK. Apparently that is not what I wanted to say.
I wanted to talk about life evolution. The more we go to the past, the more we come near a GrafPort who is the base of everything draw in a computer.
Think Apple and Oranges (and everything else) have a dna… And that dna is based on an unique ancestor who changes during the billions years that ellapsed since the beginning.
Darwin explained that too 
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What led to troubles in understanding may be in the flow of this thread:
1: you asked how to automatically colour words in a TextArea.
2: @MarkusR showed a possibility
3: you told you don’t think the IDE does it that way (which, by itself, doesn’t discard Markus’ answer)
4: @KarenA answered that the IDE probably doesn’t use a TextArea (meaning it’s expected the IDE won’t be coded like we would for a TextArea)
5: you went in the direction of telling a TextArea is drawn on screen like everything else and life evolution.
While I have no problem with that, the 5th argument was misleading to me, as it doesn’t really answer the earlier steps.
AFAIK, they are using TextEditCanvas, like BKeeney’s Formatted Text Control.