It is an early hire but it doesn't have to be shares. It often has shares because a lot of these guys can't pay market rates.
10% is a little high. Even for a 3 founder startup without investors, it's about 33% each. "Founding engineer" isn't exactly a CTO or tech lead title. It sounds like position where you take a similar workload to the CTO, but at a higher salary, lower responsibility.
Contractor roles are common as a lot of startups don't want to deal with the legal implications of having a full time employee, firing costs, etc. It's easier on a low budget too, just to contract enough MVP and then spend a couple months raising the first round of investment, or marketing the product.
I liked contractor roles because the time/pay ratio is excellent and the risk/equity is as well. Do note that 90% of startups fail. Joining something with even 50% equity might not be a good deal, especially if you've never worked with the founders.