It is important to differentiate between requirements and suggestions. Holland America enforces the dress code that should be enforced, the explicit requirements such as, "collared shirt and slacks," while inviting those who want to dress in more rarefied attire to do so. When they talk about what people "usually wear" they're not posting a requirement. Furthermore, they're describing the average over all their cruises, but attire is generally more casual on their Alaska itineraries (and more formal on their longer cruises, greater than 15 days).

There are still a lot of very crotchety people who have been cruising on Holland America for decades who cannot handle the fact that their personal preferences regarding attire no longer prevail in society, and so you get thousands of posts on Cruise Critic about how someone's meal was "ruined" because the man at the next table was in a polo shirt. Just ignore it. The requirements are clearly communicated, and are what most of the mainstream cruise marketplace wants in terms of requirements, regardless of what those bitter about change have to say about it.