No, they absolutely used it as a ground-attack incendiary and have admitted as such. They were not flushed out by being illuminated, they were flushed out with choking smoke and burning shit raining down on their positions.
Even if they did only use illumination flares, there are considerations against using them in civilian areas in ways that can start fires or otherwise cause injury to civilians.
The legal issue is moot because the US was not an adherent to these laws until 2009.
My understanding is that the drill is fixtured in position in procedures as delicate as this, so that it really can’t move and drill anywhere except where it needs to. Likely why Dad thought (wrongly) that it was harmless.