is the creator estopped from making any future claim of copyright in the work?
It depends on the extent to which the author's conduct is indicative of abandonment, which precludes a subsequent claim of copyright.
Black's Law Dictionary defines abandonment as "[t]he voluntary relinquishment of possession of thing by owner with intention of terminating his ownership, but without vesting it in any other person. [...] 'Abandonment' includes both the intention to abandon and the external act by which the intention is carried into effect. [...] [T]he intention is the first and paramount object of inquiry [...] the lapse of time may be evidence of an intention to abandon, and where it is accompanied by acts manifesting such an intention, it may be considered in determining whether there has been abandonment".
Establishing the element of non-vesting is straightforward: Attribution to someone the author knew was dead reflects that ownership is not to be vested in any other person.
The external act is the author's voluntary attribution to the long-dead artist.
Intention to terminate ownership seems harder to prove, "although the lapse of time may be evidence of an intention to abandon", Id. See also National Comics Publications, Inc. v. Fawcett, 191 F.2d 594 ("We do not mean that the "proprietor" [...] may not evince such a consistent disregard of his right to copyright [his work] as to justify the inference that he intends to "abandon"", brackets added).
Whether or not the author profited from the false attribution seems inconsequential. The question of abandonment is decisive because to abandon entails "intent of never again resuming one's right or interest". Black's Law Dictionary.