Thanks for the comment. If you would have read through the entire article, you would see fees mentioned in the "pitfalls" section.
I am glad you brought up the NBBO, as it is an extremely important regulation for newer traders to understand. However, the comment on mergers and consolidations still stands...just look at the Philly Stock Exchange: https://en.wikipedia.org/wiki/Philadelphia_Stock_Exchange
Arbitrage money is net of fees between exchanges so your ELIF showing $5 profit is kind of irresponsible giving people false information. If you are concerned about your users do mention the fees in bold letters above fold.
On your question on exchanges in US, well read this:
https://www.investopedia.com/ask/answers/08/security-market-...
And it doesn't even mention IEX:
https://en.m.wikipedia.org/wiki/IEX
So no not only two exchanges in existence.
And I mentioned NBBO because it nearly removes arbitrage opportunity not because there are only "two exchanges". But if you want to change your goalpost to focus on M&A, I digress.