There are two scenarios that one may be trying to express by the formulations that are at stake here.
There are two different composers: Ken and Kann. The actual composer was Kann, but the booklet mistakenly states that it was Ken. That can be expressed by saying that the track was mistakenly credited to Ken. The preposition tells us that the mistake concerns where the credit went: it went to the wrong person. Note, however, that even in that scenario we would not say that the 'composer is mistakenly credited to Mr. Ken'.
There is only one composer that is relevant, but there is some confusion about whether his name is spelled as Ken or Kann. The correct spelling is Kann, but the people who wrote the booklet mistakenly thought that it was Ken. That can be expressed by saying that the composer was mistakenly credited as Ken. In this scenario, unlike the first one, the credit went to the right person; the preposition as tells us that the mistake concerns the manner of giving the credit to that person.
It should be noted that 'the composer was mistakenly credited as Ken' means something different from 'Ken was mistakenly credited as the composer'; the latter formulation would fit Scenario 1.