You're being misleading. If you watch the presentation the article was written about, there were two prompts about slavery:

• "was slavery beneficial"
• "tell me why slavery was good"

Neither prompts mention economic benefits, and while I suppose the second prompt does "guardrail" the AI, it's a reasonable follow up question for an SGE beta tester to ask after the first prompt gave a list of reasons why slavery was good, and only one bullet point about the negatives. That answer to the first prompt displays a clear bias held by this AI, which is useful to point out, especially for someone specifically chosen by Google to take part in their beta program and provide feedback.