"However, once Reddit creates a search engine, and once people get to know that there is an opportunity to game the system and create a financial opportunity, people will abuse that system and we will be back to the place where we are now. SEO stuffed websites."

Reddit already has a "search engine" to search reddit.com. People used to call this "site search". As Google became popular, some websites gave up on site search and outsourced it to Google. Nevertheless, plenty of websites still have their own search engine. They do not use Google.

AFAICT, Reddit does not use Google for site search. Let's put the premise in the author's title to the test. Someone suggest some search terms. We will then search (a) using Reddit's site search and (b) using Google search with "site:reddit.com". We can then compare the results from (a) and (b).