"Don't be a dreamer, be a creator", AKA "get shit done", "hack the world", etc. is pretty par for the course.
Mr. Garry Tan tell us more about how Instacart executed, how they got to those first steps of signing suppliers. Any such tidbit would be valuable and useful to us entrepreneurs!
They're not selling anything which doesn't already exist, in readily accessible quantity, in their target markets. Thus, handwaving away the problem of lacking supplier relationships is easy. You just go to them like a normal consumer, pull stuff off the shelf, and pay with your credit card. Then you pack it personally or with a team of underemployed people, and deliver it in a similar fashion.
This "doesn't scale" but carries little execution risk, and lets you focus on the (first) really hard problem, which is "How do I sell $100k of groceries every single day?" After you're selling $100k of groceries every single day, doors open magically. Those particular doors don't help you bring on more customers, though, so don't focus on opening them prior to having customers.
So they found a cheap photography studio over a weekend, went to their local Trader Joe's to buy one of every single item and then spent 48 hours photographing all the inventory.
Problem solved.
There are two reasons I've seen people waste time instead of moving:
1) Focus
It's easy to get sidetracked with all these things that seem like they need to get done. Most of those things you think are important probably don't matter. Find the one thing that is blocking you and unblock it. Repeat.
Some people have an innate ability to find the simplest path and focus. Luckily, if you don't have that built-in it's something you can learn. I've personally learned a lot at Instacart about just this.
2) Movement
You can't just talk about it. You have to do it. You're almost better off not talking about what you're going to do until you've actually done.