The main differentiator between Amazon and everyone else continues to be Prime. Having people paying $120/year to come back and shop on their site is an advantage that nobody can compete with. Shopify could try and compete by creating a similar membership service, and perhaps they will, but at this point I'm skeptical it would gain much traction. The main reason people use Prime is to buy commodity goods with fast free shipping. In this case I'm defining commodity goods as those where the supplier isn't that important and the alternative is driving to a local store and buying the same thing.
Shopify is almost at the other extreme: the sites that sell goods on its platform must differentiate themselves with niche offerings and consequently, I'd bet that the average Shopify customer makes purchases a lot less frequently than the average Amazon customer. People shop on those sites because the local stores don't have what they are looking for.
Another reason I'm a bit skeptical about the "Shopify is a threat to Amazon" narrative is that at a certain scale it seems that stores would leave Shopify and just manage their own websites and inventory. So Shopify is sort of a business incubator that is extremely valuable when a business' scale is limited. But the few that become huge hits would likely leave to save transaction costs, so Shopify would lose that business.
Totally agree with the Wal-Mart analysis though. They seem to be all over the place. I sometimes wonder why Wal-Mart doesn't at least try to start a subscription service. They would definitely have to undercut Amazon on price and eat losses for a while, but I don't understand the idea of advertising a no-subscription required model when that model clearly works so well for so many other businesses. The most obvious issue is that they are at a huge disadvantage in distribution center build-out, but why couldn't they start regionally in a couple of densely-populated areas and expand from there? Price the service aggressively and maybe give members discounts on groceries or other in-store products, an area where Wal-Mart has a massive advantage over Amazon.