What is the signifigance of Drones being recognized as life forms?

As you noted, if it picks up Engi, fluffwise there's no reason they shouldn't pick up drones. This game doesn't try very hard to make biological sense; don't get caught up on it.

Gameplaywise, while it may feel misleading to lase an enemy to find him completely freaking unharmed, it's more misleading, and more dangerous, to teleport into a ship and be caught unaware by more bodies than were on your sensors.

Slug crew sense sentient beings no matter if theyre made of flesh, rock, crystal or technology, that includes high-tech drones and Engi as well as rock men.

They will probably sense the enemy ship's AI, too, and they would sense beings of pure energy and ghosts, if they existed, but not holographic projections or dumb fighting drones from our 2012 age.