GMAT Sentence Correction (SC) | Re: Less than 35 years after the release of African honeybees

Dear Mike,
Thanks for help and support as usual

Actually I hoped you can shed some light on the following grammar construction in choice C:

In lessthan..........since African honeybeeshad been released ................had migrated

As far as I learned, what comes after 'since' is 'Past simple' or 'certain points such as: month, year..etc' NOT 'past prefect', especially that the main clause of the sentence has 'past prefect'.