Bit of a stretch to claim MIMD/vector processing covers that particular product [0]; maybe they're going after that because it's got a Mali 400 GPU (in which case I'd just refer them to ARM's lawyers).
I once had a similiar situation, where we were going to be sued if we didn't pay up. I sent back a letter asking them to explicitly identify the infringing item; never heard back.
[0] http://web.archive.org/web/20160909142123/https://www.sparkf...