A lot of people say a lot of stuff on the internet. Some of it is pure malarky.
There are people on this forum who are a lot more into knives than I am but I'm sure they will eventually chime in and call that notion malarky too.
The only type of knife that I'm aware of that is virtually impossible to sharpen are ceramic blades. I believe they do need to be sent to the mfgr for sharpening.
Interestingly, on the info sheet that Shun puts in their knife box they make no mention at all about sharpening. They say:
"Congratulations".. bla bla.. you bought a real fine knife.. bla bla
"Warrantee" .. bla bla.. FOR COMPLIMENTARY SHARPENING OR REPAIRS PLEASE FILL OUT THE WARRANTEE FORM...
"Cutting surfaces", "Washing", and "Storage". You can imagine the "bla bla" in those sections.
I do believe that if you are the original owner and have filed the registration they will sharpen the blade free of charge for the duration of the warantee (lifetime of the original purchaser). I seem to recall that they even pay the return shipping.
To stretch that to a statement that the only way to sharpen a Shun is to send it to them is stretching the truth, I think.