I just don't see it that way.
I mean, first of all, it's a metaphorical 'licking of wounds' for the Necrons retreat. I find it perfectly reasonable to think that they might not have the necessary resources available to defeat the Blood Angels after taking on a hive fleet. Yes, all of their damaged brethren would have phased out - doesn't mean that they'd have enough left on the battlefield to continue what would turn into a protracted campaign.
Regarding the Blood Angels - all branches of the Imperium (including the Inquisition) 'abandon their duty' in this manner -all the time-. Alliances of convenience are made tons of times in 40k fiction...because realistically it's impossible to expect Imperial forces to try to kill everything non-human at all times ever. There are simply times where it's more tactically sound to ally with the enemy of your enemy, for example.
Think of how many times Marines have 'allied' with Eldar, for example. Or in one of the Ciaphas Cain novels, an entire book is spent where the deployed IG forces are doing everything in their power to -avoid- starting a shooting war with the Tau, who have all but conquered the planet through political guile.
Also, no offense taken.
I know this particular example is a point of contention for a lot of people, but I've never really had a problem with it...just like I've never had a problem with transport vehicle scale in 40k, for example (i.e. Rhinos are actually larger than they probably need to be, not smaller), but that's something that a lot of people seem to find odd. I just like getting my 2 cents in on these points from time to time.