"it goes out of its way to obscure the license terms"

They make the license terms very clear. Their license page highlights the differences to the text of the Apache 2.0 license. The only change is that you can't sell the engine itself. https://defold.com/license/

"intentionally deceptive" "there's a big fat nav item at the top of the site"

The Defold guys have released multiple components on Github with the MIT license. The fact that a navigation item is named "Open Source" does not say that all or most of the software from this company is Open Source. For example, the fact that the Apple developer site has a navigation item named "Open Source" similarly doesn't mean that all or most of Apple's software is Open Source.

"Everything about this screams"

You're attributing unverifiable negative intentions to the Defold developers.

This, the last HN thread about this, and other similar negative feedback create an incentive to avoid the term "Open Source" in order to steer clear of potential negative publicity for innocent misunderstandings and different opinions.

But they also have an incentive to keep the words "open source" in a sufficiently prominent position on their website to ensure that search engines list their page in the results for the search term "open source game engine", since it's a relevant result for almost all people who search for that term.