That's not what he's talking about.
GUI is the interface of the World Editor that allows people to create triggers without having to manually write the code - basically, you click on things in the UI, it makes a nice graphical representation of the trigger and then once you save the map it converts it to a code readable by the game.
What he was talking about is adding more functions (Events/Conditions/Actions etc.) to the GUI, so that you can do more things with just the built-in trigger editor instead of having to rely on importing custom systems or using custom code.
And I agree, it's a bit of a shame that not a lot was added. I think that adding things like a damage detection system or leak removals for unit groups/point or quite a lot of other things is long overdue, because while old modders will be fine without them, it would be really useful for newcomers - and you know a lot of folks are going to try making their first maps with Reforged.
P.S. And yes, they've added some natives that allow people to tinker with the UI, so you weren't wrong about that