Your right. ENDCOMMENT is more like a delimiter. Comparable to ENDTEXT, which is also shown as regular text. And comparable to ENDIFF which is ... show as a command.
Well, I agree with Rick. It should be colored like a command. And while you're at it. If you don't mind. I think the lines between COMMENT and ENDCOMMENT should be colored like comment ... meaning, green.
Totally different situation (as is ENDSWITCH, which is also a command). IFF / SWITCH mean "execute commands within this command", not "ignore everything until you find a trailing ENDCOMMENT / ENDTEXT".
I agree with your explanation of the difference between these keywords to the TC interpreter. At the same time I don't think your insight into the internals of TC addresses the right issue. I'm guessing Rick is saying that the current syntax highlighting coloring scheme is surprising, if not confusing, instead of helping. That is the issue, not whether there is an internal difference in the interpretation or execution of ENDCOMMENT and ENDIFF.
Coloring doesn't serve the execution of a script. It is a way of showing sections or brackets or other things that should be grouped or balanced. Like indentation. It exists only for the human programmer. It supports us in our limited capacities of visual perception.
If coloring confuses, it does the opposite of what is is intended to do.