He was at fault as you had the corner by a few car lengths. Even in multi class racing it is the responsibility of the overtaking car to do so safely. The car being overtaken is supposed to continue to drive their standard line so that they are predictable. Which is what you did. He should have waiting to pass till after the corner or attempt to pass on the outside with his superior speed and downforce.
He's in the wrong and a dick for claiming otherwise. Hope he got car damage that ruined his race but knowing the netcode... he probably made it out just fine.