In a nutshell … nope! Developmental editing is very different to copy-editing and proofreading.
Developmental editors help authors shape their stories (addressing things like concept, plot and structure, pacing and character development; and stylistic and technical execution, such a deciding on tense, viewpoint, voice …)
Copy-editors address the mechanics of the sentences (fixing grammar, punctuation, spelling). They’ll make sure your words are consistently presented, too – for instance, eradicating inconsistency between ‘dreamed’ and ‘dreamt’ or ‘co-operate’ and ‘cooperate’.
Proofreaders check that the copy-editor hasn’t missed anything. (They will have – it’s inevitable when dealing with so many words!) They also check that the layout of the text on the page is correct and presented well (so no awkward hyphenations between lines, for instance, or end-of-chapter pages that contain just a single word).
There’s another type of editing, though, and that’s called line editing. This is also known as stylistic editing. Sometimes the boundaries between copy editing and line editing get blurred. Sometimes the boundaries between developmental editing and line editing get blurred.
This is because there is no universal consensus as to what each editorial service entails. As soon as you start thinking about the artistic (rather than mechanical) elements of a novel, the solutions become more abstract and less prescriptive. Every novel will require something different, and every editor will bring something slightly different to the table.
Line editing looks at the artistic expression of the writing at the sentence and paragraph level. It might address similar issues as a developmental edit (e.g. plot, pacing, character) but it will do so on a smaller scale. For example, a line editor might suggest tweaking a phrase a character says in dialogue if it doesn’t seem to fit their character, but they won’t give an author an extended analysis of that character. Line editors will edit a manuscript to help improve the flow, rhythm and style of the sentences – in the context of the bigger picture.
So in essence, there are three main kinds of fiction editing: big-picture, stylistic, and mechanical. The one in the middle has crossover with the other two, but mechanical and big-picture are far enough removed from each other that there’s very little crossover in knowledge needed.
Which means you don’t need to know how to copy-edit or proofread to become a developmental editor because copy-editing and proofreading deal with the mechanics of the sentences whereas developmental editing deals with the big-picture story elements.
(You might also find this post useful: How Much Training Does a Fiction Editor Need?)