Seems like it's free for open-source projects, so the pricing should apply to commercial users. Sort of similar to what Qt does.

If it is under Apache 2 license as the repo claims, very few commercial projects are going to have an issue with using it under the open-source terms. You can't say "this is open-source under license X, but you can't use it commercially!" (well, you can, but you are creating your own license that's not compatible with open source projects either). Qt solves this by being under license(s) not all commercial users want to comply with, and selling support etc.

It seems what they are selling is not the software, but services it can use to manage exchange between users etc.

There's a server component that's not on Tanker's Github, so commercial users probably can't start using the SDK without telling anyone.

They can legally use the SDK under the open source terms without paying if they find a way of using it without Tanker's servers. For the server access, they need to pay, thus that's what they're selling.

In contrast Qt is selling licenses to the code, not access to a hosted service.

The client side code is open source. The server hosting aspect costs money. Open source is not the same as free.