The only thing that a banner "by scrolling down you opt in to something" means according to GDPR is that the people who wrote that banner are liars.
Quoting GDPR definitions (Art 4.12), '‘consent’ of the data subject means any freely given, specific, informed and unambiguous indication of the data subject’s wishes [..]'.
If the site owner wants to assert that the user provided consent, it's up to them to demonstrate that they fulfilled all these criteria. If they assert that a particular action (e.g. scrolling in this case) indicates consent, then they have the burden of proof to convince the regulator or the court that the action was unambiguous, that it's clear that reasonable people would only take that action with the intent to opt-in, and not because of some unrelated reason such as wanting to read the content below. The legality of various opt-in 'dark patterns' has been tested in EU courts already before GDPR, and it's not considered legally valid if it systematically misrepresents the actual user intent and wishes.
The appropriateness of any technical measure can be trivially tested with a user survey - get 100 random people to use the site for 5 minutes, and after the "opt in" action is completed, ask them whether they know that they "opted in", whether they know to what they opted in, and whether they really intended to opt in or not - and if not, then your method for "collecting consent" does not work and the things you recorded - actions, clicks, even physical signatures in a 'meatspace' setting - do not give you any legal permission whatsoever, they're meaningless.
The one thing they can do with that banner and scrolling is to cover their information requirements, where they have the right to do some thing even if the user disagrees (much less opts in), but they are required to notify the users that they are doing that thing. But there it makes all sense that the user can choose to delete that banner if they want to, it's like the notifications in supermarkets that they have cameras - they're required to display them, but the users don't need to read them if they don't care.