Hey, author here.
First off, there are absolutely no guarantees with this stuff - it can work for some people and not for others. Something you should be aware of is that when you use a turbo SIM to unlock your phone, it has to be in your phone all the time along with your MVNO SIM. If you just take out the turbo SIM and the MVNO SIM, you should be able to continue using your original Docomo SIM. So to test whether it works, you’ll need the turbo SIM plus an MVNO SIM. If you don’t want to sign up for a contract with IIJmio, Mineo or U-Mobile etc., you can just as easily use a tourist SIM to test if it works. Likely worst case scenario is you wasted a few thousand yen on a turbo SIM and a tourist SIM.