This seems like a great solution. If you use a stable subtop that won't move and can be secured firmly to the legs and aprons, you should be able to build the desk without the black cross aprons and have it prevent racking, etc.
Then attach the solid walnut top to the subtop allowing for movement of the solid walnut top through elongated screw holes or what have you. Depending on which way the grain runs, I'd fix it along either the 39" or 33" back edge, allowing for the wood to move in and out towards the user, rather than causing the top to push in and out against the wall. (If the desk is going to be placed in the middle of a room, that's less important, but someday it might be against a wall, and so I think it would still be a good idea.)
I find Paul Sellars interesting to watch, & he is undoubtedly talented, but I think he partly exists in a different universe of his own making. There are just too many comments he makes that just don't ring true.
I do that all the time too. We all make mistakes.
What doesn't feel like good netiquette to me is when people change their answer based on a subsequent post. For example, someone asks a joinery question and user A answers "Nails". After a while user B answers "Glue". After seeing that, user A edits his post changing it to "Glue", without saying anything about having edited his post. So after that, user B looks like he copied the answer from A while it was the other way around.