One should understand how their tools work but there's no such thing as doing that without understanding things across the abstractions involved and at least a good portion of what happens under them, Deno or not.
It works the same way you use a bundler instead of assembling your own and so on and so forth down the tree. The farther down the tree, the less focus you should give your understanding to, but that's not an excuse for giving no understanding below the first layer.
This is why I'm not overly concerned about AI taking our jobs. Those who do not RTFM* are doomed to rewrite it in blood, toil, tears, and sweat.
* Or, in the absence of well-written documentation, the source code, the decompiled object code, or at the very least inspect the end result.