Last night I got to either 920,000 or 980,000 (I was 20k off 900k or 1M, can't remember which), and died! So close to the survive to 1,000,000 achievement. Anyways, it didn't unlock the 10x multiplier achievement. I have no idea why. I assumed I unlocked this once I passed 700,000 or 800,000 for sure, but it didn't happen. Just a heads up that it may take longer than you expect.
A possible reason for this, and I'm not sure of the dynamics of the game so this is open for discussion, is that I used up all of my bombs just to get to 500,000 without dying, and maybe by killing so many enemies without getting points for them caused the multiplier to lag behind the score more than otherwise. I repeat this is only a guess.