The immunity thing was implemented in TQ:IT. Probably also implemented in later TQ patches. Anyway +pet damage items didn't exist in the original TQ, so certain nature skills were more powerful then. Plague was one of these.I could swear I saw this happening in my game, but I'm not really sure about it now.
If you did take Refresh, note that you can simply keep it at level one and let +skills do the job. You don't actually need it to completely deal with Call of the Hunt's cooldown, you're only interested in it helping you cast Call of Hunt more often. I don't think you need to keep Call of the Hunt up all the time anyway.