"Seed" has shifted. In the early days of YC, you came in with an idea and tried to build something people want. Note that it is called "Demo Day", because you would literally demo your product.
Today, it's just so much easier to start a company. You can try out an idea in a weekend, so the 3-month program of YC is focused on growth. As you said, they are preparing companies to raise seed capital, and the bar for that has risen considerably.