It doesn't matter because the DB had committed interference first. If it was only Lamb extending his arm, then it would have been offensive pass interference, but the DB had already grabbed and impeded him.
Again, it should have been a no-call IMO because the DB committed interference and then Lamb committed OPI to get away. But I think the official had already made up his mind to flag the DB for his interference before Lamb pushed off and made the catch.
You can't ignore the DB's interference just because of the push-off by Lamb.