We live in a world where people have the attention span of a few seconds to wait for a website to load before they give up and try another one (see Amazon/Pinterest research on income lost from 1 second of loading time).
The same applies to Youtube videos, you either capture the attention of the viewer within the first 10-30 seconds or people are just going to click another video on the feed.
Like it or not, this is the reality of today’s media.
Watching the 20-25 minutes for a full episode is a relatively big investment and some anime connoisseurs are experienced enough with the medium to tell if something is worth continuing or not. However, I’ll admit that it is not always applicable: Examples exist, counterexamples also exist.
tl;dr: Just let him/her judge whatever he wants. It’s just one comment/opinion out of many. It shouldn’t affect your own judgement :)