@The Kitchen Sink,
What the developers are saying is that they can change the "engine" of Avast by making "little" changes in the antivirus database, without the need to update the complete program.
The database is usually updated twice a day, and if there is a critic problem they can update it more often too.
The "engine" evaluates the real need to completely scan bit by bit a certain (type of) file.
Avast Team came to the conclusion that certain files are not worth checking, at least for now. "Videos" are mostly skipped, but maybe, one day, without even announcing it, the developers could add some type of file to the regular scan.
So, unless the "zero seconds scans" are happening "everywhere" (no matter the type or size of files), then you can trust that everything that needs to be scanned, is scanned (if your settings are correct).
To be perfectly clear, the "zero seconds scans" indeed happened to me in the past, but it was clear that it was happening "everywhere". I didn't find a "solution", but a "workaround". I downloaded the "clean uninstall" utility from Avast, started "Windows Safe Mode", and clean it.
After a clean installation, the "zero seconds scans" disappeared, together with my concerns.
There are additional reasons to "zero second scans", like for example the "cache" that Avast can use to improve its performance.
So, if you are in doubt, "clean uninstall" Avast, and "clean install" it again (following the correct steps).
Other than that, I think that Avast is protecting you successfully.
I hope this answers your questions, but you are free to ask again and for sure you will get your answer.