Most mathematicians say (nowdays) that a primenumber is a number that can be only divided by two numbers that is 1 and itself. 1 can be only divided by 1 which leads to the fact that most mathematicians don't consider 1 as a prime anymore. Which is a good idea since the meaning of a prime is that it can only be divided by two numbers, 1 would be a irregularity in that matter which would undermine the meaning of prime numbers which is being only dividable through two numbers. So yes and no, many mathematicians considered 1 as a prime number. Most mathematicians nowdays consider 1 not as a prime number. And that is that.

To make it more clear again. 1 would be only dividable through 1. All other primes are dividable through two numbers which is one and itself. Cause of that definition 1 can't be a true prime since it's just dividable through a single number that is 1. It makes no sense to categorize numbers and then allow a irregular into it. That's why the view changed and 1 isn't considered as a primenumber anymore and that is also now shown in the game.