It’s not in what is specifically said, it’s more the attitude that everyone should and can be vegan. For a variety of reasons, some of us just aren’t able to be. Most normal vegans accept this and don’t worry about it, but the fundamentalist ones have been known to harass people with various conditions not conducive to being vegan.
For example, some of the more fundamentalist vegans will demand that bipolar people not take their meds because they contain lactose. They also go so far as to try to take away service animals, saying they are “enslaved”. These are just a couple examples.
From personal experience, I have problems absorbing iron so I have to eat a lot of it in order to avoid becoming anemic. The pills don’t work at all. So my diet has to include red meat. Not only am I not obligated to become vegan, I’m not physically able to.