You correctly stated that the carbon layup (i.e. the number and orientation of plies of carbon used) is likely to be lighter in areas where the frame isn’t typically as heavily loaded. You also correctly identified that CF is anisotropic, meaning that its material properties can be made to vary by direction, unlike metal tubes.
I heard on one of the podcasts I frequent that at least one carbon frame model was made so light that the manufacturer explicitly warned against sitting on the top tube. I don’t recall which model, nor even which podcast or which episode (for reference, it would have been Marginal Gains by Josh Poertner, or the Cyclingtips podcast, or the Nerd Alert podcast which is a separate podcast under Cyclingtips’ umbrella). I also believe it isn’t a general practice to warn against sitting on the TT. Consider that we aren’t putting our full weight on the tube, we are typically sitting on it while stopped with one leg on the ground.
I don’t repair carbon, and I don’t wish to discount the experience of your local carbon fiber repair person. That said, be aware that he experiences what statisticians would call survivorship bias: people come to him because their bikes are broken. He doesn’t observe all the CF bikes where the owner did sit on the top tube, but the bike didn’t explode. I think you would generally be justified in sitting on your top tube. If you have a really light CF frame, I would use more caution, perhaps a lot more. I would also use more caution if you’re heavier than average.
This is a digression for interested parties: here’s the Wikipedia page, which
has a story that’s relevant to my description. In World War II, the US Air Force was examining bullet holes on returning B-17 bombers. They initially added more armor to the areas that got hit the most. However, Abraham Wald and colleagues told them to add armor to the parts that did not have bullet holes on the surviving planes. Here, the areas were the engines and cockpit. In hindsight, it seems obvious that hits there would disable the bomber entirely, and the USAF would not be seeing many bombers that took cockpit or engine hits and survived to tell the tale.