Does prem bu mean ms 65?

Depends.

Premium BU usually refers to a coin that is above average uncirculated, usually with pleasing (or no) toning and is a bit more subjective than MS-65.

Its possible that something could be advertized as Premium BU but only grade an MS-63 or MS-64 because the scratches/nicks that decrease the grade aren't easily visible on the coin (usually because they are hidden by toning), or a premium BU coin could grade higher than MS-65.

It also depends on the dealer. Some dealers very sparingly use premium BU on coins and the coins that they call premium BU really are the choicest of the choice. On the other hand, there are some dealers that use premium BU all the time and apply it to coins that are average uncirculated.