Yes filter sponges do contain BB ... in fact just about every surface in your tank contains BB ... and filters contain the most. Sponges do contain a healthy amount of BB ... maybe much as ceramic media since sponges are so porous offer a lot of surface area for BB to colonize.
Honestly ... Unless it's falling apart ... I'd give it a good rinsing with tank water and put it right back ... even if it is unsightly.
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.