Not attacking all the other candidates at once may have been a winning strategy for Trump, but maybe it was more the logic of the campaign that determined Trump's attitude towards Cruz, not anything Cruz said or did: Ted Cruz wasn't going to win New Hampshire (or Florida), so Trump could concentrate his fire on other contenders.
The account was part of a larger discussion of the failure of other Republican candidates to take Trump down in the primaries, perhaps because they didn’t take him seriously enough. The post-mortem was atypically hostile, with Trump aides basking in their win and accusing the Clinton camp of being sore losers, while Clinton aides decried an election they said was won because of a resurgence of white supremacy.
That sounds like a very biased account. Did the Trump aides come in "basking in their win" and "accusing the Clinton camp of being sore losers"? Or did the Clinton aides acting like sore losers to begin with simply produce a response in the Trump team?
I believe the sore loser thing didn’t come up until the Clinton operative said that Trump only won because he appealed to base racist instincts in the country and Steve (Race) Bannon was the perfect example of that. That’s when Kellyanne shot back with something like well, you know what? You lost. ;-)