He is referring to the fact that Kiwi is based on a mix of different Chromium versions of 2019. Chromium does new release almost every month. It's difficult to keep track of Chromium modifications with such large forks (and Chromium had lot of performance issue at the end of 2019 too with stuttering and lags).

UCBrowser for example is an engine of 2017, Samsung Internet from 2019, etc