This is not an urban myth - it's just a misquote/misunderstanding.

I'm assuming Russel's double negative was accidental, and he meant, "If you do your code in 'c' - moving from one micro to another is a lot less painful"

I would qualify that as, "If you do your code carefully in 'c' - moving from one micro to another is a lot less painful"

It's the same as the old "code written in assembler is quicker & smaller" chestnut: writing in 'C' allows you to make you code much easier to port - but the mere act of choosing 'C' as the language does not of itself guarantee portability!