U.2 is more popular in servers. M.2 is common because of laptops. It is used in servers mainly for boot drive or where someone is gaming the cost difference of enterprise drives. There are form factors like Intel's ruler if you really want density that bests even custom M.2 carriers for servers.
Another point the author may not be aware of, most server SSD sales happen as parts of bigger integration deals. The SKUs for many/most U.2 devices for instance might never hit something like Newegg or Amazon but might be moving in massive volumes at Supermicro or Sanmina or the like. This is in contrast to say 3.5" hard disks which spent most of their life forwarded by pushing consumer demand and the only difference in an enterprise disk may be some firmware knobs.