The statement, "not available for third party licensing," may mean Playboy doesn't have the right to license the cartoon. Although we are not privy to the arrangement between the magazine and the cartoonist, a case involving the artist, Patrick Nagel, revealed that until July 1979, Playboy acquired rights from artists by printing the following legend on the back of checks:
Any alteration of this legend agreement voids this check. By endorsement of this check, payee acknowledges payment in full for the assignment to Playboy Enterprises, Inc. of all right, title, and interest in and to the following items: [a description of the work].
An assignment versus work made for hire. In the Nagel case, the court of appeals determined that this language referred to an assignment, not a work-for-hire. One difference between the two types of ownership is that an assignment can be terminated after a number of years while a work-for-hire cannot. For example, transfers of ownership made in 1965 could be terminated in 2021 (see 17 USC 304). Therefore if Playboy used a similar legend in 1965, and if the cartoonist's estate terminated Playboy's rights, the cartoonist's estate may own the right to license the cartoon.
Or maybe they don't want to deal ... Alternatively, the cynical members of the Dear Rich Staff wonder if Playboy just doesn't want to bother with small licenses such as yours and they use this statement as an all-purpose rejection.