Yes that is true. In particular, the original camera team walked.Last I've heard, the set was not safe before the incident ... the union people walked off the set in protest, got replaced (because of course) and the tragedy happened after the walkout due to the replacement people NOT following proper safety procedures (because they weren't union people who "knew the rules" surrounding such things and WHY those rules exist the way they do).
It’s entirely possible the replacements were non-union, or that the entire production was being done under a non-union contract but that last is very unlikely. It is likely that much of the crew were “getting their days” in order to qualify for full membership. I don’t know the particulars but I have friends in Santa Fe and I’m sure I’ll hear the ugly details soon enough.
For the record, I’ve been a member of the camera union - the International Cinematographers Guild - for about 15 years. It makes me really angry that this happened.