My guess would be that the Egyptian slave thing stems from another time where it was exotic to travel and only adventurers would go to foreign lands and bring b

My guess would be that the Egyptian slave thing stems from another time where it was exotic to travel and only adventurers would go to foreign lands and bring back people from there (slave or apprentice - would like to see the original kit). And there was a lot of mysticism around exotic countries, so having such a person on your mystery show (alchemy) would simply increase the effect. Yes, it sounds stupid today, but we are talking about a time before the invention of Google, antibiotics or routine international flights. I'm sure other than that PoCs generally had less access to chemistry kits, probably mostly because they couldn't afford them. But I don't think that Chemistry kit was the issue.