How to unlock horse masks in RDR2?

Solution 1:

Horse masks are only available via Outlaw Passes.

The guide that you located was teaching you how to equip a horse mask earned by an Outlaw Pass.

Here are two quotes from the same website you went to:

Most players’ first interaction with Horse Masks is via the Outlaw Club rewards.

The Outlaw Club grants players 70 new tiers of items to unlock as they play the game, so there are plenty more cosmetics to enjoy.

This second website more specifically says you can get them as part of the Outlaw Pass only:

You can get Horse Masks as rewards via the Outlaw Club, a new season-pass type item added to the game recently.