Why does this sound like clandestine propaganda to discourage veganism & vegetarianism?

No, there's nothing clandestine about it. They're just stating a fact, which is that vegans (especially) often have a B12 deficiency. But as they point out, you can take a supplement tablet once a day and be just fine. Yes, vegetarianism is healthIER overall than eating a carnivore diet, but it has its own perils that you should be aware of.