I do feel that it might be possible to learn something from the rec solo training, if we could find out what exactly it is that they teach.
Of all of the SDI/TDI courses I teach, the Solo Diver course is the hardest to describe in response to this type of query. I've just finished teaching a Solo course today and it was very different to any other course but then each one is always very different. This is because I base each course on the student. Part of the course is to push the student a just little bit outside their comfort zone to introduce them to stressful situations which they can then learn to deal with. This is used as an example of "deconditioning panic". Of course each student has a different comfort zone and has slightly different things that will push them just outside of it. As a result the instructor has to use a lot of judgement to gradually build up the stress to the right level so that the student is outside their comfort zone but not so far outside that they stop learning.
As Jax says, it teaches a mindset more than anything else. For some people it is just about looking at their diving differently. For other people it is about learning about skills and techniques that they did not previously know. For others it is about having an instructor look at their techniques and procedures and providing feedback on what improvements could be made.
For this reason it's almost impossible to state "what exactly it is that they teach".
I know this sounds like avoiding the question so I'll try to be more helpful. After each course I ask my students "what did you get from the course". Below are some of the answers, starting with the one I had from a student (who was also a professional diving instructor) just over an hour ago.
"I've realised my skills are not as good as I thought they were"
"I didn't realise that some of these equipment issues were so important in an emergency"
"I've learnt more in this course than on any of my other courses"
"I never had any intention of diving alone but this has helped to make me a better buddy"
"I've realised that a pony cylinder might not always be enough"
"
I know that even if I never solo dive in the future, it has made me a better and safer diver, I would recommend it to anyone."
"Now I know exactly how I want to set up my kit"
"I've only just realised that even though I had a buddy I was effectively diving solo"
"I can't believe how quickly I managed to get beyond my comfort level"
"There really is no excuse for running out of gas"