Allah Almighty can show any form to any creation. If He decided to show the mountain a finger-like object that caused it to immediately collapse, then so be it. No one can describe Allah Almigthy.
I don't know where Ibn Kathir got the "finger" part from, but be as it may, the tafsir would not be wrong if that indeed happened. But Allah Almighty is LIMITLESS and INFINITE. We have no clue what His Divine Realm is like. No one can describe Him.
So how do you know the Quran is from Him?
Look at the Glorious Quran's Numerical and Scientific Miracles which the Almighty promised to reveal to all of mankind to thoroughly prove to them that the Glorious Quran is from Him:
www.answering-christianity.com/ac20.htm#linksDo not derail yourself into too many questions that attempt to describe Allah Almighty. You can't with your human logic even answer things like;
- Which came first, the chicken or the egg?
- How could you when you were a microscopic being grow to thousands of times bigger in size today?
Logically these things don't make sense. Yet, they're materialized here on earth.