I don't really understand this. Its very common when starting a new topic to start from a secondary survey and then dive into primary sources - often recommended by the secondary survey.

You are also missing a distinction between doing philosophy (what the post is advocating) vs doing history of philosophy, which secondary literature has different roles:

Doing philosophy:

Platonism about mathematics has had proponents and developments since Plato, its quite likely that that secondary survey provides links to more recent primary sources than Plato himself, which you would otherwise be ignorant of.

Doing history of Philosophy:

Diving stright into ancient primary sources without some supporting secondary literature could leave you confused and without some kind of historical context.