But, hypothetically, if true it would not be racist to say it.
As you say, we do not have absolute proof, but it might be true.
You can not be certain it is untrue, so it might not be racist to say it.
Supposing you truly believe it true.
Is it racist then?

BP Lord Ahmed does believe culture has caused a predisposition.
Is he a racist?
Likewise BP Mohammed Shafiq.
Is he a racist?
Likewise Jasmin Alibhai-Brown, a Muslim and half Pakistani.
Racist?
Would you like to tell her that?!
And, if anyone is convinced by their statements, do they become racists?