It is a little bit different in the new book, and a lot different in the epub book. The Platoon itself is set up the same, however each Squad has an Army List Entry, and aside from a slightly encompassing border, provides no link to them being ONLY available via an Infantry Platoon.
This is a little bit different from the 5th Edition codex in which the Platoon and its squads were held in a huge completely encompassing black field.
If you are building a Platoon, this is correct. However, nothing requires these Squads to ONLY be purchased via Platoon. Search as you might in the codex, you will find nothing on this. Remember, when something "may" be done does not mean it "must" be done or "only done" that way.