it's not a lie at all
it's too easy to say a character is lying when it suits people.
deidara says sasori is stronger than him, that's a fact
and to confirm even more this fact , it's clear that sasori dominates deidara, he considers him a capricious child and threatens him on several occasions.
even in edo tensei, we see sasori threatening deidara, he has a relationship of domination with him.
the deidara sasori relationship is not at all equal to equal, whether alive or in edo tensei, it's always sasori who gives the orders, threatens deidara , and deidara who's a bit of a capricious child.
you don't need to know anything about human relations to see clearly who's the boss and who's the child.