As you said, experienced engineers have been dealing with CSS browser support issues for years, so as a habit try to avoid using the latest shiny new features.

Also, experienced engineers do refresh their knowledge from time to time, but you might have caught them between refreshes. Remember that these engineers are also working 40-60 hours per week producing work output in addition to periodically refreshing their skillset. And depending on the environment they've worked in previously they may not have had the ability to use the latest features of whatever technology. Maybe they were working on a legacy app that didn't support ES6, for example. That would be less common now in the days of Babel, but there was a time a few years ago when it would have been reasonable for a working JS engineer not to be familiar with every detail of ES6, as an example.