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I've never heard any mention of a remote that will link via rf (zigbee) to a Comcast set top box. I presume the older non-X1 would be the lower code set and the later one would be for the later one. so many functions for X1 are moving to voice control (go forward 3 minutes, for example) that an IR only control would not be able to perform.
the only non-X1 remote method that has ever been discussed is IR. can't help beyond that. if you can take the actual other-brand remote and operate it directly at the set top box then you know the issue is your extender.