I don't understand the issue...you should paint in whichever program or medium your most comfortable in. You can import background images created in other programs, or scan or photograph practical art, so it really doesn't matter much where you create them.
TVPaint has a lot of very nice tools and if you like painting with it, that's obviously the most convenient option. If you like Krita, another excellent paint program, you should use that.
In my case, these days I do a lot of my painting in Procreate on an iPad because 1.) I like the program's gesture-based UI a lot, and 2.) painting on the iPad is more convenient for me when I'm away from my desk. When I'm done, I send the final image to dropbox so it will be waiting for me (and TVPaint) when I get home, I may do some additional touchups or processing in TVPaint or another paint program before importing it to TVP, depending on what the image needs.
So use what you like, not what somebody tells you to use. (Well, unless it's a job requirement, of course.)
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