I am compelled to write all the list of things why it will not work :) unfortunately I don't have that much time in my life. Don't take it as putting you down, I would love to see such thing working and live in real world, so look at it as problems that would need to be solved while aiming for grand solution.
First, there are wrong incentives at play, good guys currently don't need that as they mostly are employed and look for jobs passively. Good guys don't care about certificates and you want to certify good guys so your certificates mean something and can build trust. Weak guys will be spending money to cheat their way around just to get passing grade, which will cost money to prevent.
Then you have hiring managers that will not use this exam as good enough just to hire a person unless there is big company that would do the same. So you have chicken and egg problem, because no one will trust your certification and to get the trust someone would have to use it.
My idea is that you cannot effectively hire at scale. I could hand pick maybe 5 devs/year for my company, getting to real numbers in my small company I can see that we can hire maybe 2 developers per year. When We hire someone it worked for me with simple questions and attitude check. I don't see a way to hire 100 good developers in 1 year, that is not possible. Maybe at a scale you can hire 10-20 good developers a year but you have to have 3-5 people involved full time - and ideally those 3-5 people should be nice and technical - where in the end you don't want to use them only for hiring because those are probably your best developers with soft skills.
For that competency matrix - that is something you can do when you have developers inside of the company and you can observe them for at least 6 months. Good luck getting that in practical testing setup for 2 or less hours. I also find those kind of matrices terrible and not useful for evaluating people or projects. I work on a project where we were not fitting in anything of what was in the matrix .. yet we were delivering good quality for business all the time, while teams that were stuck on their "competency matrix" trying to improve meaningless points ended up disbanded and not working anymore in the organization.
First, there are wrong incentives at play, good guys currently don't need that as they mostly are employed and look for jobs passively. Good guys don't care about certificates and you want to certify good guys so your certificates mean something and can build trust. Weak guys will be spending money to cheat their way around just to get passing grade, which will cost money to prevent.
Then you have hiring managers that will not use this exam as good enough just to hire a person unless there is big company that would do the same. So you have chicken and egg problem, because no one will trust your certification and to get the trust someone would have to use it.
My idea is that you cannot effectively hire at scale. I could hand pick maybe 5 devs/year for my company, getting to real numbers in my small company I can see that we can hire maybe 2 developers per year. When We hire someone it worked for me with simple questions and attitude check. I don't see a way to hire 100 good developers in 1 year, that is not possible. Maybe at a scale you can hire 10-20 good developers a year but you have to have 3-5 people involved full time - and ideally those 3-5 people should be nice and technical - where in the end you don't want to use them only for hiring because those are probably your best developers with soft skills.
For that competency matrix - that is something you can do when you have developers inside of the company and you can observe them for at least 6 months. Good luck getting that in practical testing setup for 2 or less hours. I also find those kind of matrices terrible and not useful for evaluating people or projects. I work on a project where we were not fitting in anything of what was in the matrix .. yet we were delivering good quality for business all the time, while teams that were stuck on their "competency matrix" trying to improve meaningless points ended up disbanded and not working anymore in the organization.