Holy Knights have been randomly name dropped several times since the end of Wano already, without anything suggesting they're mean to be secret. Back during W7 we were actually told CP9 was mean to be a secret group the moment they got introduced.
To be fair, they likely aren't as well known as the marine since their role is more specific than the "world police" so most citizens will never encounter or hear about a a Holy Knight in their life but I don't see any real reason to assume their existence is actively concealed aside from agenda (aka WG lied about admirals being their strongest fighting force because they don't want anybody to know they actually have a strongest force for some reason).
To be fair, they likely aren't as well known as the marine since their role is more specific than the "world police" so most citizens will never encounter or hear about a a Holy Knight in their life but I don't see any real reason to assume their existence is actively concealed aside from agenda (aka WG lied about admirals being their strongest fighting force because they don't want anybody to know they actually have a strongest force for some reason).
But instead, the WG declares the Admirals to be its strongest military force. Lol