OO / FP are paradigms. Monolith / microservices are architectures. Paradigm and architecture are orthogonal. I guess the author used "philosophies" because the

OO / FP are paradigms. Monolith / microservices are architectures. Paradigm and architecture are orthogonal. I guess the author used "philosophies" because they are mixing pears and apples, so the most specific word that can be stretched enough to encompass those concepts is "philosophy".