Bitmain is announcing that if BIP148 succeeds, then they are essentially going to mine a new SHA-256 altcoin instead of Bitcoin. If you don't do anything, it'll be as if mining power dropped somewhat, but you'll otherwise be completely unaffected.
Big words from their side, but no, I clearly don't believe them. They will not go away from the precioussss Bitcoin for a (worthless!) altcoin they'd create.
This is only a "political declaration" (which I've learned that's the nice way to say about a lie). However, I will not be neither happier nor sad, I think that this means nothing and nothing will change.