That is basically what you should never do, in any topic, unless it's some esoteric topic and a prior top-level explanation would help.
It's not like Plato is Quantum Physics.
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.
That is the only advantage of secondary sources. Telling you that guy X and Y wrote about Z. But if you want to learn about Z, go read what those guys actually wrote.
If you don't understand a particular concept, word, or whatever, then do a research in secondary material available.
>literature could leave you confused and without some kind of historical context.
Specially in recent (basically anything written in the last 50 years) secondary sources I heavily dispense with comments on "historical context" as they are full of bs, and are heavily politically biased, one way or another, not necessarily in fields like Mathematics, but in History or Philosophy, etc.
To relax from the reading, and since most (all?) of us did already have some advanced math education beyond the average philosophy student (e.g. I did major in CS, others had maths major or minor), we did a "guided by the professor" proof for the incompleteness theorem - he always wanted to do that, but couldn't in his normal lectures. That was fun.
I'd suspect I wouldn't have had such fond memories if we everyone was to read the original sources, especially since that would have been nearly impossible in the given time frame. In most seminars that's usually accelerated by having one person read one source each and do a presentation and/or write an essay. I think reading a well written essay and discussing that in a small group is the superior approach.
Of course if you want to become a professional in the field this will not replace reading some/most/all of the original sources in the long term. It is then a matter of opinion if it's better to start reading the source with or without some broad-but-shallow knowledge of the field. I prefer with, since I never read such material to "just learn what X said on Y so I pass some exam", but because I like thinking about it and put it into perspective [e.g. temporal, but also alternate ways of looking at things]; but then I'm not a professional in that field, so insert shrug-pony here.
It isn’t exposition — it’s dialectic. It’s well written and just as relevant today as then.