The discovery rule kicks in with these sorts of crimes. A shitass US Attorney (and there are plenty of them) could argue that no reasonable person would have been aware of the perjury under the circumstances, and date of Sosa's admission of the perjury would be the trigger for the limitations period to kick off, rather than the date of his original testimony.
At best, we'll get an extremely codeworded, vague, and mealy admission from Sammy that will be along the lines of, "I did things that were wrong," that won't implicate him as a perjurer.