Furring strips can be any material, plywood, 2x4, 2x6.

I think people are getting stuck on the 2x2 furring strips being the only type of furring strip, which isn’t the case. You can just use 2x2s in a lot of cases, and there is a specific product called furring strips to use for furring or whatever else you want.

Some people even buy 2x4s and rip down their own furring strips of 2x2 if the span doesn’t need the 4 inches.