'part time secretary needed, light admin duties, possibility of position growing to full time' is a completely normal job ad. Any temp agency in any city you happen to be in can give a hundred applicants if you're hesitant about hiring someone too early.
I'm not sure if this is too optimistic or too pessimistic, but I definitely don't buy it in general. Founders want to be building the product and talking to users. I can only imagine there's all kind of other crap that would be smart to hire someone to take care of.
The idea is to find the things where there's not a lot of value in having the founder do the work and find the cheapest way to get it done.
There's a few options for how to do that. You can go to a temp agency, or you can get a recent university graduate with generalist skills pretty cheaply.
That's not true for all startups, nor even most, but I've seen startups where the founder was spending more than a day a week doing data entry.