What are they making? It's a game, designed to be played antagonistically. The two players are in direct competition with each other, whatever the playstyle we choose to label them with. This game has points values for units, down to upgrades worth just a point or a handful of points. There is an implicit and explicit design principle that units are worth a value proportional to their points cost and that two armies of equal points should be roughly equal in ability.Funny thing is, the design team don't go out to build the most extreme lists, as it is not part of the philosophy the company has regarding the gaming aspect. If I remember right, when the issues around dual Lash DPs first came up it was something that the design team didn't think anyone would take or words to that extent. How is it the designs teams fault if the game is played in a way they have stated that it is not intended for.
If the game is designed to be played antagonistically - and when I say that I mean head-to-head, not that the players are being antagonistic to each other - then the job of the designers is indeed to make it balanced as possible. This may or may not be intended for tournaments, but unless you throw points values out the window yes I expect the game to be balanced. Not every game has points values, Inquisitor for example does not. It is close to impossible to play Inquisitor "competitively" and if you do the campaign will likely implode immediately unless the GM is rather creative. I love Inquisitor, I think it's a fantastic game and it's a very good example of an antagonistic game that is not competitive.
If the designers do not "stress test" their games then, simply, they produce a crummy product. We would expect computer games and other programs to be stress-tested and not just given a buggy product and told "yeah, you're not using it as intended" if you find issues with the program being used without user modification. And software companies have lots of people who do exactly that.
The game places constraints on the players. Sometimes loopholes get through and in all fairness it's hard to see how the designers could possible have predicted that. The "chosenstar" tactic in WFB for example takes a bunch of very reasonable units and turns them into a horrific combination. I didn't see it the first time I read the warriors book indeed it didn't become popular until many years after the book's release.
But... Lash? They didn't see the possibility that people might want to take two HQ units? It's not like it's some loophole like the Chosenstar, it's basic FOC!
Ultimately you can play a narrative game using units that are correctly pointed. It is hard to play a competitive game with units that are incorrectly pointed. If the game is balanced then everybody wins.