It just refers to them using the bom as a "data source" - it doesn't specify that they use forecast data.

For the suburb I'm in right now, the BOM forecast tomorrow is 21-25 degrees with a 100% chance of rain. The Apple forecast for the exact same area is 17-24 degrees with a 70% chance of rain. That's a pretty big difference. Even the current temperature is wrong, the BOM's data is showing that it's currently 24 degrees here, while apple is saying 26.

The only BOM data I've ever seen on the Apple weather app is weather warnings, and when there's a severe thunderstorm warning it'll show up on my lock screen widget as well as in the weather app with a link to the BOM warning text.