I worked for IBM who suggested a similar self rating scheme. Just before announcing redundancies.
Fortunately, when I pointed out they need some kind of control candidate or manager agreement to stop people exaggerating, they listened.
Unfortunately they had me mediate, and I had a couple of dozen one on one meetings where I would more or less give people a polite correction of their skills.
A year later, I too was redundant. I had to submit my own skills matrix.