I'm not sure if they count the dvd's the same as the count the cd's in a multi set package... And Billboard did post that LIB sold about 58,000 units, but those are soundscan numbers based on actual scanned sales. The RIAA combines sales and shippments to stores, record clubs or any other outlet that sells the music, so it's a combination of both. So in actual sales it's above 50,000 units, but in total sales and shippments it exceeds 100,000 units allowing it to be certified platinum.
This is from the RIAA site:
RIAA.COM
Music Video Longform
GOLD
Quantity:
50,000 units
Requirements:
*Defined as a title that has not been theatrically released prior to distribution.
PLATINUM
Quantity:
100,000 units
Requirements:
*Defined as a title that has not been theatrically released prior to distribution.
MULTI-PLATINUM
Quantity:
200,000 units
Requirements:
*Defined as a title that has not been theatrically released prior to distribution.
*Recertified at 100,000 unit sales level.