I'm going to say again.
While I don't know their reasons were, if it was up to me, these things are not just about gear.
If I had to pug a random stranger to do Denova, I would not take a tank who has not done it before.
That's certainly the raid leader's prerogative, but I think it's a poor choice. Tanks are hard enough to come by already--by only taking tanks who already know the fights you're locking new tanks out of the system. If you know that a number of your pugs are new to the place and you don't want more than two or three newbies, I can understand that. I don't think telling tanks they need to roll an alt, level to 50, gear up, and learn the fights as a healer or dps is a very good strategy long-term if you plan to pug.