If we had to publish a journal paper to justify every effort at being good, we would get nowhere.
First of all, having just scanned the referenced paper, I have to agree it hasn't followed the scientific method. There is no "placebo" group, there is no monitoring for recidivism for 10 years, etc. However, the mistake is not in volunteering to educate prisoners, it's only in the approach to writing about it as a science paper making conclusions that can't really be backed by the limited data available with the "experiment".
Helping people who need help - that's one of the principal components of human instinctive behavior (although there will differing interpretations of who is deserving).
Helping prisoners reform by giving them some fulfilling purpose is actually a no-brainer and already widespread. E.g. True post civil war story - The Professor and the Madman (2019). Furthermore, high school, tradesmen skills, and higher education courses, have all been a staple of prisoner reform for as long as I have been alive.
It doesn't have to scholarly or education related either. After I was bitten by a stranger's dog, I ended up watching a lot of YT videos about prisons where prisoners are allowed to take on badly behaving dogs and train them up to the point where they can adopted. Their testimonies - as they explain the profound and positive impact on themselves of helping another being (the dogs) and being loved by them in return - are truly moving and impressive. Anybody watching those videos would get all the proof they need from the emotions evident in the prisoners words and faces - e.g., "I have never been loved before".