Is this true?

Most of them are still around because it used to be so difficult for them to keep running. The industry joke was, "When you take delivery of your new Rolls, they present you with your keys and a bus schedule." The truth was that most owners were well enough off that they could maintain them regardless of cost. Even at that, few of them racked up the miles the way our everyday drivers do. 

Of course, since BMW took them over, they are more reliable than ever. 

Perhaps, but they might actively be used as doorstops.

I'd believe it. It says just less than 75% are still in use and I'd find that believable. As JK mentions, as an extremely expensive automobile, owners are likely to use it sparingly and keep it maintained. As a standard or lower-valued vehicle ages, it is more likely to be less well cared for and more liable to become derelict.