I can confirm with a very little spelunking that the Mac app is a real native AppKit/Cocoa app. It appears to mostly be written in Swift, but also uses a fair amount of Objective-C. They've said publicly that the Windows version is written in C# and C/C++. Not sure what they did for Linux...

from the website:

> TablePlus is a native application. We are using Swift, Objective-C, C/C++, Perl for OSX, C# for Windows.

It's a native GUI on the Mac.