Well flying through those extremely thick mines doesn't look like anyone, no matter how trained, could pass.
Perhaps there's a thing about mines, that if you transmit the proper code with your beacon, you can render them inactive for your ship, and that's how Junkers that got supplied by such beacons by the Gallians could pass through the mines.
Also, there could've been a small passage through the mines, but then surely someone else would find it too.
The uncharted jumpholes theory is also viable, makes sense, although it does seem a bit far fetched.
<span style="font-familyalatino Linotype">
<span style="color:#000000">All morons hate it when you call them a moron.
alatino Linotype">