are anonymous credentials a type of zero knowledge proof? Are anonymous credentials (such as IBM Idemix) a type of zero knowledge proofs (ZKP)? if so what type of ZKP are they?

The papers such “Security without identification: transaction systems to make big brother obsolete” don not reference ZKP.

My understanding is they both refer to similar topics.
I would rather call them a type of blind signatures. Since signatures stem from non interactive ZKP you can say that they are related.