@MichelleElynHogan, were you responding to a particular post here, or making a more general comment about the study itself? I ask, because my reading of the article is that there were objective measures developed and used to test individuals' heat stress response. My view is that measuring the point at which an individual's internal temperature started to rise would have eliminated any subjectivity on when they were unable to 'maintain a relatively stable core temperature over time'.
There are several considerations that might create some uncertainty about what is dangerous, such as the study being conducted on young fit individuals undertaking what might be seen as relatively undemanding activities indoors, compared to walking outside, potentially without shade, which is what most of us do on the Camino. I would suggest all of these things indicate one should be even more conservative about assessing when it becomes dangerous to walk.
On this, the US National Weather Service suggests that Heat Index values can increase by as much as 15F (~10C) where the activity is conducted outside without shade - conditions that most of us face walking on the Camino.
All this goes to the conclusion that across much of northern Spain over the next few days, the maximum daily heat indices will be above the point that can lead to dangerous heat disorders with prolonged exposure and/or physical activity in the heat.
It would be unfortunate if someone reading your post were to take it that they could rely on some subjective view of their own heat tolerance to keep walking in such conditions, rather than avoiding walking, or planning to take a break during the worst heat of the day.