I'll partially decent from Chris here: the 8.x SRND is a victim of Cisco's internal support policies. They require that the BU test specific product combinations (not merely the protocol) to limit the scope of what TAC must support. This policy is also why Contact Center Express only supports Microsoft Exchange even though it utilizes normal IMAP and SMTP: the BU must test a product, not just a protocol. The document is really talking about the Domain Functional Level, for which there is a 2008R1 and 2008R2 distinction but the BU couldn't be that specific. Can you run 2008R2 (which is x64 only) with a 2008R1 functional level? Yes. Will it work? Yes. Can you call TAC if it breaks? No, at least not officially.
One interesting point to make here is that the distinction has been removed in the 9.x System Guide (they moved this part from the SRND in 9.x). The lack of distinction here works to your favor: both 2008 R1 and R2 would be covered by TAC. Again, it really means the Domain Functional Level, but they had to call it out by product.
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