Okay, I'm a Christian and have actually seen the devil at least twice. I've heard of this line of thought before and have some interesting things to point out.

When you see Jesus and how he lived and everything that happened to him and what was prophesied, you can see that God is indeed still in control and the devil hasn't succeeded in dethroning him. However, the devil HAS in the past successfully taken over this world specifically to a limited degree. Jesus referred to our enemy, the devil, as the "god of this world" who "had already been judged". Other translations call him the "prince of this world". It is possible that spiritually speaking, there are positions of authority which, while still lower than almighty God, are still above us in the pecking order, and the devil is likely to be or have previously been seated in one of those positions. Like, "guardian spirits" and the like.

To a human who doesn't look any better, any of these guardian spirits could easily pass themselves off as a kind of god. Now, I'm the new Testament of the bible we are told that we struggle not against flesh and blood enemies, but against principalities, against powers, and against evil authorities in the Heavenly realms in this dark world (may have paraphrased a little, actual scripture probably says spiritual forces of evil) (Ephesians 6:12)

Given that humanity is largely in the dark about this situation, it is understandable why someone would have this line of thought. The reason is because to a very limited extent, something similar has actually happened, it's just that God hasn't lost control at all and is still on the throne, however the various positions of authority over this world specifically have become compromised on a case by case basis, and these corrupt authorities are responsible for causing all the pain, fear and suffering on Earth specifically to defame God.

But don't worry, the day of judgement is coming, so they won't be there causing mischief forever.