Whether Daring Fireball is making the logical assumption based on Apple's previous behavior and iPod touch price points, or they're privy to something a little more concrete, that's what Gruber is saying in response to the Zune HD's pricing of 16GB/32GB for $219/$289.
(Though currently the second gen 8GB iPod touch is $229, not $199...?)
Since the iPod touch has two NAND Flash slots for the iPhone's one, it typically gets double the upper-range of storage for the same year's iPhone. Thus, the 32GB iPhone 3GS almost certainly means a 64GB iPod touch this September.
If accurate, then, there goes Microsoft's purported pricing advantage, storage-wise. (If the iPod touch does indeed have built in camera and video capability, or other so-far-unleaked features, it could again be a case of Microsoft shipping a spectacular Zune, 1 or 2 years too late...)