They are fairly rare. They are the least printed bill [1] and aren't included in a typical cash register, so just randomly stumbling upon one in the wild is a pretty rare. Sometimes people think they are fake because they've never seen one before. The only reliable way to get them is to specially request them from a bank or order them from the government. Also, because of the novelty, people tend to collect them, at least informally, so they don't tend to circulate much either.
When I worked fast food, a customer paid with a few $2 bills. It wasn't strange to me. But when I tried to hand them as change to another customer, they looked at me like I was an idiot and demanded that I pay them in real money.
Before then, I was cashing my paycheck at a bank. I'd occasionally ask them for $50 of it in $2 bills. The bank had them approximately half the time, and it was fun to pay for things with them.
How do people know that they are rare? I don't even know what bills exist in my country but I know they keep changing them now and then. I wouldn't be able to tell whether it was rare or just a new bill I haven't seen before.
The US has a small set of valid bank notes. It's not like other countries that have multiple issuers, each with their own schedules for when to update designs.
On top of that, this isn't just a design that a person hasn't seen often, it's a denomination. Think of the 500 EUR notes, they were unfamiliar to many people.
(In Europe no one would bat an eye about any € coin. Only the now-discontinued but still legal 500€ notes have this effect)