Isn't divestment just one side of the coin? The other side would be moving those investments in cleaner and more renewable sources of energy which should have a clear impact.

Yep, I'm fairly sure the jury is out on this, see Bill Gates' recent comments, https://www.ft.com/content/21009e1c-d8c9-11e9-8f9b-77216ebe1...

My thinking would be if they divest from Oil and Gas, they will invest this capital somewhere else such as Nuclear, or renewable resources.

non-paywall: https://archive.fo/PGRcH

Why is it that this thread is full of people saying constraining the supply of something (financing) that is critical to the actual supply of a product will have no effect on the price?

The banks can raise the interest rate on financing. That has a huge effect on something like this where large investments and equipment and labor projects are required to get oil extraction underway, especially with commodity pricing fluctuations and the impending threats to fundamental demand from EVs and alternative energy.