It'll really depend on what has been represented to MSFT/SUN.

e.g. If at the time of the deal they were all sent documentation saying, SCO are now our agent deal with them for all licensing , then yes almost certainly. (Though here still the issue of the precise rights SCO has sold, as you still wouldn't be able to hold Novell to the deal if SCO has overstretched the rights Novell has to sell.)

On the other hand if everyone was written to saying, SCO are our agent for all issues regarding licensing, except for extending licensed rights or selling new licenses, then no they wouldn't be legitimately acting as agents in this case and MSFT/SUN would have known this from that letter.

Given the APA doesn't extend those rights to SCO it is quite possible that the latter case is true, on the other hand the press releases from the time would tend to indicate the former ("We've sold the licensing business to SCO"), but without knowledge of the communications to licensees I think we'd all be guessing as to if agency is an issue here.