The view I have on offline achievements is that if there's no evidence that you earned your achievement offline in an illegal way, it's fine. 90% of the people who earn achievements offline make is obvious that they hacked because they unlock achievements out of order when it's not possible. I know there are some games where you can do that like Halo 4 but there are games where you definitely cannot do that and if it shows up on your gamercard like that, you're deemed a hacker/gamesaver and most likely will get reset because of it. And of course there's the online achievements unlocked offline dead giveaway.
It is possible that you could unlock enough achievements offline to reach 1M but a decent percentage of games are really hard to unlock certain achievements. Plus there's also the fact that Stallion83 has at least 3 types of Xbox's (1 NTSC, 1 PAL and 1 NTSC-J).
The problem is that if you screw up which console you have your profile linked to and play offline on a console that isn't connected to the internet and that doesn't have your profile linked you can lose achievements. When I first moved to Australia I didn't have internet for 6 months so I gave my profile to my nephews so they could use my Xbox Live Gold account and play all the games I bought. During that time I played on my own system at home offline and amassed like 10,000 achievement points. When I finally did get internet all those achievements disappeared. So now I have save files for later in the game which means when I play a game I get achievements for stuff way out of order. This happens a fair bit when you don't handle offline play correctly and transfer your profile. It isn't cheating and explains why some people are missing earlier achievements. Hence, don't be so quick to judge and call someone a cheater. I wouldn't even know how to hack achievements and don't care enough to do so.