To answer the question posed in the title, we don’t need to go back to the definition of an oblique asymptote, we just need to know what a function is.
The relationship between horizontal and oblique asymptotes is as follows: horizontal asymptotes are special oblique asymptotes. Therefore, every horizontal asymptote is an oblique asymptote, but not every oblique is horizontal.
This can and should be used to shorten the counting of function asymptotes.