Well, that’s not true. But in practice if you are doing stepped starters, you may not see as much of a difference with low or high viability yeast. Even with a stirplate, there’s a maximum number of divisions you’ll get before the yeast switch over to fermentation and/or crap out. One cell won’t grow in 2L of wort, for multiple reasons, but if you start small you should be able to step up any amount of yeast. Starting from a plated culture, you can begin with 50ml or so.
OK, maybe 2 cells… ;D