Er, I don't know about you guys but I consider that yes, this scenario:
"Here is a source of information, suitably weakened to the point that you might have an edge to deal with it!"
- You get the full reward if you skip the content and kill it using base game mechanics outside of any interaction - If you do engage and interact, you have to make at least one save... and if you fail that save, you get a Game Over (most likely)!
Yes, indeed, that IS bad game design. It's terrible game design. It's inviting players to engage with your content, rewarding them for not doing so, and brutally punishing them for trying to with a complete reset - so that they must either skip the content, and get the reward externally, or they must save scum and reload UNTIL they pass the game-over save and can continue.
That's Bad Design.
In some cases I would agree with you, and there are obvious cases where using barrelmancy which you aptly demonstrated, can allow you to simply bypass all encounters and conflict.
In this isolated case it simply subverts expected RPG behavior (TALK TO ALL THE THINGS) by placing you into a fraught situation if you do so. The game in this case warns you about the encounter you are facing, and you have an interaction with the Illithid Thralls first who demonstrate that they are mind-controlled due to their interaction.
However you resolve things with the thralls you now have the option of engaging directly with an entity that has demonstrated the ability to use manipulation and powerful psionic mind-control abilities to cause harm OR you can smash it's brains open with a rock/sword/whatever.
The Illithid obviously wants you to engage directly. So given the way this is set up and the nuance around it I would reject that this qualifies as bad design.
AS a DM I would love to give players this scenario to deal with, and I think it works much better that way since the Illithid can only engage with one person at a time using its mind-control ability. It gives the other players a chance to notice what is happening and then they can take action if they see their companion getting taken over.
