It's remaining old description and already not applicable (applicable to limited range of commits, so kept for anyone accidentally built / installed the affected old main, maybe). See the last part of README (README.md that are shown on browsing the URL I've linked previously).T-Aoki, in the link for Ken Dechui's overlay, he writes that it doesn't work well in 15.0-CURRENT. Obviously, 15.0 is no longer CURRENT, but are there still problems with it. (ChapinTokyo, apologies for hijacking the thread, but it's only one question. ゆるして くださ)い。
I'm now on stable/15 and having main (aka 16-CURRENT) on another physical SSD, his overlay is working fine on both now for months. So no worries.