I watched the video and it seems to be made with synthedit (by the looks of the “IDE” he showed). That doesn’t look very serious to me, but more like a hobby project. Or have things changed regarding this? I’d expect something like JUCE or custom to be used for a ‘high grade’ software synth.
Wow, this theme is repeated then. I did not see it. If any moderator sees this, then he can join it with the other topic.
I will only say that, for € 1, nobody should doubt it. If you really just want to collect that amount (without any “hidden” business behind), I hope this man sells 2 or 3 million copies. Or more.