Is the search for a specific n-gram the same like a string search?
Well, as you state the problem, it is true that the search for a certain sequence of strings/words is the same as looking for the corresponding n-gram. 

However, keep in mind that an n-gram, when you use it for ML, (often) is represented as a factor. So a certain sequence of words or strings, is thought of carrying valuable information. Like „John Holmes“ is different from „Sherlock Holmes“, when it comes to, e.g. identifying book titles or covers.

In general, an n-gram is just a certain sequence of words/strings, which carry more information than only one word/string.