I am not convinced that you are competent enough to be an engineering manager at a big company with oversight from an experienced manager, nevermind at a startup. You've revealed a cascading set of failures here that are 100% your responsibility and then have the audacity to caricaturize your well intentioned junior engineer:
1) Having 5 months of runway is an existential problem that your CEO's fundraising should have addressed 7-12 months ago
2) Lacking a proper code review process ensures that you'll pay pounds of prevention long term instead of ounces of prevention, which over the long term is not a frugal usage of your already scarce engineering resources
3) Your senior engineers should be having the conversation with your junior engineers about the tradeoffs of heavier vs lighter forms of code review and CI process. Unless...
4) You hired a junior engineer as your early or first employee without a senior engineer and are unwilling to do the necessary work mentoring them yourself
You are responsible for each of these failure modes. Who made the decisions to get the company to a place where it only has 5 months left of cash? Who made the decision as a startup manager to let other managers take it there? Who made the choice to hire the junior engineer? Who made the decision to forgo proper code review standards?
It sounds like your junior engineer has a sense that they are on a sinking ship. Maybe they're right. You're one of the captains. Turn it around, or cut your losses and learn from your mistakes. Saying this as someone who has been a startup manager, I think it is a profound breach of the duty of leadership to not do so, which is what it sounds like you are doing right now.
EDIT: In case you need some pointers or some food for thought, I think that another HN thread that's also on the front page could be a great place to start:
Remember that management is not the same as IC work at a higher seniority. It is a fundamentally different vocation. You really need to internalize that to be effective and it is tricky.
https://news.ycombinator.com/item?id=23427110
https://news.ycombinator.com/item?id=23429785