It's absolutely the case. Around 60% of seats are available for "pre-selection". After that, all others (as well as exit rows & bulkheads) are held back for assignment on the day of travel. They're not held for frequent fliers - it's just how Virgin manages to allow families etc. who book late to still be able to sit together on the day.

Exit rows can only be assigned AT check-in (physically there), others can be selected online 23 hours 59 minutes and 59 seconds before you fly