Nope, it's the client itself, and for many affected users, wasn't a problem until the update which caused the issue, and has been an issue ever since in spite of multiple users reporting various demonstrable issues causing it and several problems which don't match up with obvious causes but can be and have been medically verified to be linked to the update to Steam.
And seizures are a common symptom of epilepsy, but not the only one, with flashing lights being only one of several potential photosensitive triggers, there are plenty of people who are fine to play this and other games but not able to safely use Steam.
And just because you know someone whose migraines don't put them in hospital, doesn't mean that's consistently the case for everyone who experiences them. You should do some research before trying to pretend to know the science behind this. I'm not a hypochondriac, I've worked in medicine before, and I know what doctors have confirmed about my situation.