Web-based payment for Nebula access instead of App Store - expected?😅

I wanted to describe my experience using the Nebula app, specifically through the promotional link that led me to their page. The offer looked very attractive, and all I had to do was take a short test and then pay a fee to get access to personalised recommendations. I went through the process, paid, and waited for confirmation through the app. However, nothing appeared in Nebula, and then I realised that the payment had not been processed through Apple. It was completely web-based. Can you imagine that? Because this is the first time I've heard that such a thing is even possible. It made me wonder - isn`t Apple`s policy quite strict about in-app purchase requirements for this kind of content? I didn`t see any warning that I was leaving the Apple ecosystem. Everything looked seamless, so I assumed it was integrated

There were no errors, no double payments, but now I`m not sure if I should trust such purchases in the future. Has anyone encountered similar processes in other apps? I would appreciate any explanations or advice from those who are more familiar with the company`s policy🙌