That's definitely a dark pattern but I think it's a good idea to have different styling for different buttons and the one that's most commonly used is the one that should be more eye catching. In this case, skip should be the one with a background but the point stands that one should look different from the other.
The problem is not which of the two is the default, the problem is that only one of them looks like a button.
It's good to use different styles. Symmetry is bad when you want people to make a choice.
But in this example it doesn't look like there is a choice at all.
It's just a dark pattern that almost all companies (including Apple and Microsoft) have started to use, where they de-emphasize certain options to the point where people don't even realise that they have a choice.
As a user I disagree they should look different. Maybe the designer wants to funnel users down a specific path, but these aren't my preferences. "Skip" is not a rare, exceptional choice here.
I think the good idea would be to give this "Skip" button at least a darker gray background (which once pressed changes into blue) upon interaction so less experienced user would know that it is still an option that can be selected. Without it, this screen suggest that only action is to press visible "Invite Friends" blue button and that is an example of dark pattern design.
It might be the approved guideline in official design documentation but it doesn't mean it's a fair design for the user.
