First, you are assuming the usage of LLMs for the cleverest code in the submission. You can get help from writing boilerplate code or testcases, for example.

Second, code review is not debugging. Code review can take as much as you want, unlike debugging which finishes when the root cause is found, and you would do code review on your code anyway even if not written by an LLM. (And dually, you can have an LLM do code review).

You can also get help in the design phase and then implement it yourself.

So, depending on the problem, you can gain time, you can gain quality, or neither.