That's not true. The SAS can come from any regiment of the Army. The SBS (Special Boat Squadron) require soldiers to come from the Royal Marines. There have been talks to combined the two because of the similar nature of the missions.
The major complaint of the SAS is that they spend too much time on fundamentals. Stuff the SBS assumes that their candidates know before serving.
What is it that isn't true? I couldn't tell from your post. I've never spent time with the SBS so I don't have any personal opinions on them.