Can a scientist in the field of applied sciences claim that he has mastered all specialties to be able to write a single author paper in a given field?
Yes, of course, you can do anything you like because all branches of science blend into each other. And it's more of an achievement to have a single-author paper
For a general discussion about single author papers (with nearly 8,500 replies!) see https://www.researchgate.net/post/What_do_you_think_about_single_author_paper .