They're actually doing the opposite: in light mode, the active tab has a dark grey background and black text. An inactive tab has a light grey background and black text. Which has more contrast? The inactive tab. Makes no sense. The tab itself has less contrast within itself, which makes it look "disabled" or inactive.
Also having 2 tabs is quite common if you don't hoard tabs.
Add the fact that your eyes only see clearly towards the center of the field of vision, you are only ever seeing a handful of tabs at once, if your screen is big enough. Always having to glance over at the other tabs and think "so which type of tab color do I have more of?" is not really intuitive.
Then add the fact that if you hover the mouse over any tab it changes color to match the active tab. So now you have two tabs that look identical, which one is active?
I know, I know, with some logic and strategical thinking, you can always figure it out. But these things should be automatic and take no time, or as some say they should be "intuitive". The current design is not intuitive.