Ok, they're kind of right but retention rate isn't the full story. It's hard to give you a number because I have videos with 25% retention with a long average view duration and that have done quite well. The most consistent stat for me on videos that have done well is click through rate.
You can easily skew retention rate average just by making short videos since they will be easier to get higher retention % on. 42% is fine, I averaged 27% over the last 27 days and it wasn't a bad month for me at all. I have a lot of 10 minute+ videos that bring down my average, my shorter videos have higher retention usually but it doesn't correlate with increased success.
It's complicated because it is an important stat but it's a very relative one and I think the answer is get your audience watching for longer but obviously this is very relative to the length of your video and that's why it's hard to give you a number. My most viewed video has 29% retention and it wasn't a disliked video.
