That basic WiFi implementation merely allows no$gba to run Pokemon (it has a WiFi check immediately when loaded which used to fail and had to be patched out for the game to even run however since this basic implementation that's no longer the case) but it can't do anything practical yet and does not work for the Underground area in Pokemon or any other game or game feature needing WiFi.
If people were able to access the underground it was likely through some sort of cheat, or perhaps they converted and moved their save to the their DS just for that part.
Which version of the emu were you using? Do you use a firmware and BIOS files as well?
The only think I can think of then is that you used a much older version of the emu with a patched version of the game...
And no there is no such patch for Platinum and it's inadvisable to reuse the former as the location of the WiFi check has probably changed...an who knows what effect the patch could have then...possibly a game breaking bug later on...also the patch now that no$gba has WiFi support causes saving problems (It works fine first, but then refuses to load (black screen)).
So it all comes back to there being no solution and no solution likely coming about any time soon if ever.