Your example is just a misuse, even of the mathematical sense of this term -- he means prima facie. But there is a legitimate mathematical sense, different from the philosophical sense.
Various very important philosophers have considered math to be a priori in the philosophical sense. Plato and Kant come to the fore. But most folks have dismissed this concept, either right away, or after looking at the failing of Frege's program, and the resulting need to reframe mathematics. It is hard to believe in mathematical Platonism, and we know various things Kant said about math, especially geometry, are overstatements that cast his other ideas into question.
And outside those (now broken) frames, it seems obvious that any 'a priori' statement about primes involves a silly notion of 'a priori'. Of course, we would not expect our native intuition to have anything to say about some random number and primality. Primality itself is hard to see as an intuitive notion. It is clearly derivative on a long experience with multiplication, and not something that would just pop into the mind of a baby out of nowhere.
At the same time, there is a proper use of a priori in mathematics, that is not quite exactly what Kant would have meant by the term. The notion of continuity seems to be a priori, in this mathematical sense. Babies seem to be able to track faces through space. At a certain age, they notice it is somehow absurd for a face to simply disappear... And the standard of comparison between different actual definitions of continuity has been how well they accord with this a priori notion, which is prior enough to the facility of language that we can't express it reasonably, and we have only really annoying definitions of it that involve infinite smallness, epsilons and deltas, function preimages, the existence of limits, or other obnoxious complexities.
There is a mathematical notion of 'intuition' that is not perfectly like Kant's notion of intuition, but to which the mathematical use of a priori is related in the same way Kant's own definition of the term is related to his own notion of intuition. It reflects the overall notion of 'elegance' or 'simplicity' in math, which are somewhat unrelated to their everyday usages.