Earlier quoted context omitted.
That's what he said: you can veify the file's hash after you downloaded it, to make sure it wasn't tampered with...
The merkle root of the DAG over the chunks is not related to the file's hash. Imagine if I gave you a file out of a git repo, and the long commit hash. It's insufficient on its own to verify integrity, you need the intermediate nodes. With ipfs you can infer that it used the reference chunker and try to reproduce the DAG but there is no guarantee it will work. Better to just speak ipfs to the p2p net and get the DAG.
Git the Merkel root is a list of directories and hashs of whole files.
Ipfs, an individual file can be made up of many trees and those trees can have trees.
Example: https://explore.ipld.io/#/explore/QmbGtJg23skhvFmu9mJiePVByh...
That is a single file, but good luck verify that file without actually downloading the entire dag tree which includes the file.
Worse, that same file can be chucked and bucketed infinitely number of ways. So if I download that file and the re-add it to IPFS, I will very likely get a _different_ address. And that address will likely share _none_ of the of the nodes from the previous address. Meaning even though the same file is in IPFS, they are completely distinct and don't actually help seed each other.