It means they sold every ticket for their gigs
Seriously, it means the band has given up their ideals in favour of (mostly) big money, giving them less artistic control, cred or whatever. I don't know the band, so I wouldn't know if they sold out.
A thing to remember is that most bands evolve. For some fans that is a tough pill to swallow. They want 'their' band to sound the same, record after record, to front the same ideals as before, disregarding the fact that most people, musicians included, grow up and change their view on things. It does not mean by itself that they sold out.
If you look it up, you will find in the entry "sold out", a picture of the Dead Kennedys. They opposed the big music industry with all the empty-minded 'stars' doing nothing but to sell brands, but after they thumped their only political aware member, they hired their guns to comercials. THAT, my friend, is a sell-out. :wallbash: