Yes,this is true, for linguists. Colloquially, man-in-the-pub test, I'm less sure. you can rhyme some words in all the dialects, but you can also ask most dialect speakers about RP and they know what you're talking about because they codeshift to RP on-need. RP itself, is somewhat definitionally "correct" for ordinary mortals. Wrongly, but none the less, I am reasonably sure this is widely believed/understood.