You are probably referring to some videos I made in 2018. These are out of date now but the principle of how you can use Firebase and Webflow together is still the same. However I wouldn’t suggest anyone use this approach to build a real world production app. A quick and dirty MVP, sure, but there are major limitations with this approach for anything serious. Think of those videos more as information on how you can play around for fun.
To add to the comments about Memberstack above, the Memberstack guys themselves make it pretty clear on their website that they can’t securely gate content:
Also AFAIK they have never claimed that they can, so there is nothing to debunk there. They openly admit that it’s not actually secure.
The only way you can have real secure content with Webflow is to load that content after the page loads, and your back end will need to verify the user’s token. Which it looks like they are working on.