In general, if a vendor is saying "unsupported" it may mean several things:
- it will work but don't call us if you have problems
- it won't fit
- it won't work
- it will fry your system and the engineer installing the option
I do not know the ServerChassis, but I would suspect it is 2. - the card which is labeled for "single processor configuration" is a full-height PCIe card, so it might simply get in the way of the cooling assembly of the second CPU:

Whereas the "dual processors configuration" card is a low-profile card, taking less physical space in the chassis:

It is also possible that one or the other is not supported due to cooling issues as the guide mentions things like
Battery Back-up unit (BBU) is not supported in the SR1630BC due to thermal limitations
In general, if you hear a vendor telling you "not supported" it means something which should not be done. Also, don't be afraid of asking Intel themselves if you need a definite and authoritative answer.