Feldera provides a service. They did not design these schemas. Their customers did, and probably over such long time periods that those schemas cannot be referred to as designed anymore -- they just happened.
IIUC Feldera works in OLAP primarily, where I have no trouble believing these schemas are common. At my $JOB they are, because it works well for the type of data we process. Some OLAP DBs might not even support JOINs.
Feldera folks are simply reporting on their experience, and people are saying they're... wrong?