The big rainbow screen appears for a short moment during powerup on all RPis. If you ubigse a too old OS, eg LibreELEC 8.0 or early 8.2 versions, on a RPi3B+ it'll be stuck on that big rainbow screen plus you'll have a lightning bolt in the top right corner.
So, the description is correct, only inaccuracy is that they shouldn't call the RPi3B+ just B+ as there also was a (RPi1) B+ model.
If your 3B+ boots fine into LibreELEC this means the OS is new enough. If you then occasionally see a lightning bolt in the top right corner it means the power supply (and/or power cable) isn't adequate and should be replaced.
After a lot of frustration with PS issues in the very early RPi days I switched to using only the official RPF power supplies a couple of years ago (also bought them from Farnell/RS)- and didn't have any issues at all with them. So that's a clear recommendation from my side.
so long,
Hias