That is the only way Walt can avoid admitting it was Oswald's print. Imagine the narrative behind this line of events.
The vaunted FBI said that the smudge was Lee's palm print.
(After the evidence was sent back to the DPD but when the FBI first examined the smudge on the white card they declared that it was worthless for ID purposes) Lee Oswald had already been cast in the public eye, as a deranged killer by the authorities. So who would doubt that his prints were on the rifle even though that was simply a damned lie.