How is strength measured here? Because that sounds strange for two species that close genetically speaking to have such a big difference on how muscles work. I would expect 1kg of chimp muscle to have roughly the same strength as one 1kg of human muscle, and having a muscle/whole body mass ratio in the same ballpark. But our muscles are localized fairly differently, with much more muscles in our lower body so depending how you measure, you could get really different results.

Also, the typical industrial-world-human is pretty weak compared to what the average humans can be under the right conditions so it may bias the measure too.

The probable explanation is that they were extremely human in nearly all respects.