in real life, many "meetings", whether by chance or arranged, occur in out-of-the-way places between people on opposing sides who are either not interested in the political or social issues, or are in it for some other reason, such as money.  I can't see how even fishing vessels that chance to meet in the Strait of Mospheira would have such interactions.  The language barrier for both sides would be insurmountable to either side, unless the Department of Linguistics has some renegade students of atevi that could speak with someone from the mainland surreptitiously.  I don't recall there being a ban on "chance meetings" in the Strait.  This might be pretty far-fetched, but there have always been those people who will sell information for some perceived gain.  Smugglers, etc., for example.  Of course, Jase or Yolanda could be dealing with Sean, unbeknownst to Bren or Tabini or Geigi.

The way it was worded didn't sound like radio interception to me.  There's a whole lot about the Mospheira-mainland relationship that is not fleshed out but more hinted at in Defiance than in previous works.  Like the fact that there was an aishi'ditat warship permanently stationed in the strait.

In real life, in these situations, there are very often smugglers, and smugglers often develop their own pidgins and patoises.  And even to transfer goods officially, there must be chances for interaction and backchannels that the paidhi might not actually ever know about.