Correct me if I'm wrong, but I believe Spider-tron 6000 also only appears in Titanium difficulty, and, later on, in greater numbers.
Having said that, I've spent probably around an hour in Titanium mode, and I have yet to see a Mark II Jetpack Bot get stuck. Most of the levels I've seen them in have either no walls, or thin/limited walls, meaning they have a straight shot to you. What's more, in the numbers they appear in, trying to deal with a pack of them head-on will almost always result in your death.
So, in summation (and to reiterate my point), I fail to see how someone who had only encountered the two aforementioned, exceedingly difficult enemies once or twice could have survived through 10-15 waves of the things, especially when they've only gotten about a third of the distance twice before.