100m people appearing in the Indian Ocean in our world that needed food imports would cause some problems due to supply shock & it would take to shift food shipments and ramp up new food production(likely, we'd have to treat it like a country had their infrastructure wiped out by a tsunami or such), but I bet our world would manage somewhat okay for them and our existing supply needs (a sudden ~1.5% increase in worldwide population)...
But a medieval/renaissance tech world would have massive problems ramping up food production to support having a sudden 20+% increase in worldwide population needing food from the nearby nations (1500s Earth had ~500m people worldwide; comparatively, we would certainly have a problem with 1.5b people showing up suddenly). Japan would probably have more people than all the nations around them combined. You can't just double agricultural production in a week.