That depends on what you mean by "activates".
Anyone can mine a larger bock right now on Bitcoin Core if they want to re-compile the code for themselves. However, nobody would be dumb enough to waste all that hash power on a block that is just going to be rejected by the majority of the hash power. The blockchain will mine a couple of blocks that are smaller than 1 megabyte and the large block will be abandoned.
Same is true of Unlimited right now. Sure someone "could" start mining bigger blocks, but nobody would be dumb enough to waste all that has power on a block that is just going to be rejected by the majority of the hash power. And if they do, they're just wasting their own money and having no significant effect on the network or the blockchain. Larger "Unlimited" blocks won't realistically happen until a miner feels confident that an overwhelming majority of the global hash power is willing to build on top of his block. An overwhelming majority of the global hash power isn't going to be willing to mine on top of larger blocks until they are confident that the block reward will maintain it's value on the market (so they can pay their electric bills). So, an "Unlimited fork" won't happen unless it is overwhelmingly supported by the entire bitcoin marketplace (in which case it will be the de-facto "real" bitcoin regardless of what anyone calls it.)
So long as Unlimited is not overwhelmingly supported by the entire bitcoin marketplace it will remain a fully compatible alternative client for those that don't want to run the code released by the team that calls itself "Core".
Note that if Core decides that it is time for a 2 megabyte (or larger) block, Unlimited will NOT fork (unlike any legacy core nodes). It is already prepared to automatically accept those 2 megabyte blocks just fine. Meanwhile, anyone that doesn't upgrade their Core clients before the activation date for larger Core blocks will get left behind.