Not when they use the conjunction “so”. If they’d used “and”, then sure - there could be any number of reasons. Using “so” as a conjunction like that in the sentence gives it an equivalent definition of “therefore"

You're technically correct in your narrow focus on the conjunction "so," but you are missing the bigger picture. Yes, "so" generally functions as a logical connector like "therefore," meaning that the first statement is directly causing the second. Their sentence could be read as "Vivaldi is closed source, therefore it's harder for users to investigate," which isn't a comprehensive or precise statement on its own.

But that's a pretty pedantic take. The point that they were making doesn't rely on an exacting technical breakdown of the closed-source nature of Vivaldi. Rather, they're making a general observation that closed-source projects tend to be harder to investigate. With that in mind, the use of "so" is informal and reflects a broad conclusion that aligns with general knowledge about open vs. closed-source software. Closed source inherently implies limitations on access, which, while not exhaustive in this single sentence, still holds weight in the general sense.