He's referring to that, if at the end of 1 Marty and Jen went with Doc to 2015, history wouldn't have played out to the point where they could visit themselves as oblvious elderly people because their history hadn't happened
There would have to be a guarantee that Marty and Jen both return home safely.
I guess you could argue that since they do get home, then it is a guarantee. Couple that with the fact that changing time in BTTF uses a ripple effect, and obviously it would take a bit longer for that effect to get from 1885 to 1985 then you shouldn't notice a problem.
Basically the fact that Marty and Jen exist in 2015 and have children is enough to prove that Marty will succeed in what happens in 1885. Or at least he will get back to 1985 whether he saves Doc or not. Jen is already waiting there anyway and Doc's life takes a totally separate path that means he doesn't have a normal life at all after 1985. (Since he never returns to and stays in 1985)
So yeah, it's all based on succeeding at their plan. Success was guaranteed from the start, where if Marty decided not to go to 1885 he would have just returned to 1985 instead, got back with Jen and lived out their lives. But since he went to 1885, he did take that risk of ruining the future with Jen. Either A) he succeeds in saving Doc and returns, B) he fails at saving Doc (Mad Dog kills him, Marty escapes?) and is unable to get back to 1985 easily (If Doc didn't go back to the salloon they would have just left and Mad Dog wouldn't even have had a chance to try anyway, but by putting themselves back at risk...) since he wouldn't have a partner to put the logs in the furnace and would have had to do that himself, then climb out to the car himself. Fortunately he succeeded, but had he failed I'm sure the people, including Jen, in 1985 would have realized it shortly, however Marty himself wouldn't have of course.
Now, had Marty and Jen gone to 2015 and found no trace of themselves, you'd assume that something happened at some point. However in this case, Doc wouldn't have even come back to take them with him anyway, which would result in no problems having been allowed to occur anyway and so Marty and Jen would live out their lives as we see them in 2015. The only difference being that Marty and Jen would have no experience visiting themselves.