You’re right that there was an energy trade-off, but it was being able to run faster and longer that was more important than strength for our ancestors, who still had quite small brains (the brain of an Australopithecus is only 35% the size of a human).
Brain size developed later, probably in a feedback loop with our diet - as we began to eat more meat our brains got bigger, which made us better hunters. And hominids actually got bigger and stronger as their brains grew.
Humans evolved Wesker muscles to gain brain mass, source: https://www.nationalgeographic.com/culture/article/140527-br...
> Humans owe their big brains and sophisticated culture to a single genetic mutation that weakened our jaw muscles about 2.4 million years ago, a new study suggests.
_A new study suggests_
I don’t think you can treat these claims as categorically true. It’s plausible and probably warrants further study, like most things in biology.
Edit: I could not read the second article you linked as it was behind a paywall, but I found the full text of the original paper[1]. The paper appears to make a much weaker claim: that a weakening of jaw muscles in humans coincided with acceleration in brain size. This is certainly intriguing, but correlation does not imply causation.
