Is it a red flag if a team doesn't have tests?

No. Meta culture puts more emphasis on impact (especially impact on business and impact on products). Meta culture discourages meaningless engineering excellence that makes zero impact on business or products. That means you always need to ask the fundamental question "what does this mean to the business?"

Having low test coverage is a trade-off. It may be purely negative for engineering excellence but it's clearly a trade-off for business. Writing tests takes time. Is it the most impactful thing to do? Is there anything more impact than writing tests given the same allocation of time?

Ask the manager and tech lead(s) explicitly about this. I expect them to provide concise and logical answers to you, e.g. "This is the problem the company counting on our team to solve. This is the priority we set for the problem space. These are the goals we define to track our progress in solving the problem. Given all of these, it's only rational to focus on shipping instead of testing."

If they can't provide a good answer that's truly a red flag! Focus on the process of reasoning and decision-making. Use that to judge a team's quality.