Yes, he was. Whether Bernie was a democrat "for long enough" is disputable, but he was definitely one during his campaign and leading up to it.
The Bernie or bust people (somewhere around 25% of berners were a part of it) made up a more significant percentage of potential voters than the Ralph Nader people.
Anyway, another reason why I don't like this "Bernie people cost Hillary the election and the Bernie or bust stole it away from her" is it implies what we feared all along.
That Hillary (and her fanbase) thought was entitled to those votes, or entitled to win the election for that matter. If you are a candidate running for president, votes are something you earn. If someone didn't vote for you, that is your fault for not voting for you, not their fault for failing to see how great you are. I can run for office right now (at least on the local level, surely) but if nobody votes for me (or a minority and I still lose the election) it is my fault, and my fault alone.
This is a free country where people have many choices, and the choice is not just vote for the D or the R, even though some of you would obviously prefer that. They can vote for an independent, they can do a write in, and they can not vote at all, and all of this is as valid as anything else. People can (and should) make their own choices.
To amplify this more, I was disgusted by the way Hilary was talking a week or two before the actual votes were cast. She was definitely talking like a president elect, like she was entitled to win, she was going to win, and she knew it... before the votes started being cast. That type of behavior definitely hurt her more than helped her. You should NEVER act that way, even if you're sure you're going to win, and even if you think you're 100 times better than the opposition. This kind of behavior makes you come across as arrogant and entitled. That was especially dangerous for her because many people already thought that of her anyway.