They could pay them from the $13B endowment they have.

Johns Hopkins University has an endowment of $13B, but as I already noted above, this journal has no direct affiliation with Johns Hopkins whatsoever so the size of Johns Hopkins' endowment is completely irrelevant here. They just host a website which allows online reading of academic journals.

This particular journal is published by Kent State University, which has an endowment of less than $200 million.