That tutorial is quite old.
If you're trying to do equips, I recommend the Mix and Match workflow.
But if you just need this one simple thing, the code would look like this:
// first, make sure to add using Spine.Unity.Modules.AttachmentTools; on top of your script
var slot = skeletonAnimation.Skeleton.FindSlot("slot you want");
var attachment = yourUnitySprite.ToRegionAttachment(materialWithPropertiesYouWantToUse);
slot.Attachment = attachment;
This peels away a bit of the abstractedness but lets you know you're creating a new RegionAttachment type from a Material, and putting that in the slot.