Philippians 2:10 distinguishes between things which are "in" or "under" the earth, so probably we should be careful not to mix up this and to believe things which are in/underneath the earth (no matter as deep) are meant with these ones which are under this world.

I think so because the bible tells about more levels of the world more often (for example:three sorts of heaven, this one, which we see, the spiritual world where Angels and demons fight each other and the heaven which is God's Kingdom).

A further reason because I don't think the tartaroo is in/underneath the earth ("under" means the second heaven, not "underneath" this world) is because the bibles tells us God will destroy this fallen world at the end and makes all new (2. Peter 3:10, Revelation 21:1, 5), so the mentioned creatures in Philippians 2:10 are all which lives on, in and under the earth (on+in = creatures which are born in this world, under = fallen angels/demons which shows a lot of activities in the spiritual/invisible world too) and when this world is completely destroyed all of them have to bow their knees (when Judgement Day has come).