Just to point out: trade between subsaharan Africa and the north (Rome, Carthage, Egypt, Persia, India) was extensive long before 500AD. [Julius Caesar, for example, had a pet giraffe].
Proto-Bantu would have been spoken thousands of years earlier, and not in the interior of the Congo Rainforest (which is, of course, about the only place where there wouldn't have been any lions)., and additionally, at a time when Bantu speakers would be confined to the interior of the Congo Rainforest, making a borrowing unlikely.
Do we know which Bantu languages actually have the word? Also, wiktionary only gives Swahili as having it mean 'lion' - every other language I can find it in has it mean 'genet'.
That probably makaes Bantu origin more likely. However, lion > big cat > genet could conceivably be a recent shift, as the range of the lion has collapsed.
The one thing that makes the wanderwort an intriguing idea here is that if Bantu speakers DID reach east africa through the congo (no lions!) then they could conceivably have borrowed a word for 'lion' from east africans who were already dealing with lions. And the native east africans at that point would in many case have been afro-asiatic and would have migrated from further north, where they could have either produced or received a wanderwort.
But this idea is a bit of a stretch, and only works if the word in Bantu is limited to languages from the eastern branch, or languages that could have borrowed from the eastern branch.
You're assuming the word would be borrowed from Sanskrit, which seems unlikely.Additionally the anusvara in Sanskrit would imply that a loanword would probably contain a nasal vowel or a velar nasal, the most common reflexes of *ṃh in Indic languages (cf. Hindi सिंघ siṅgh lion). It would additionally be unusual for the nasalized vowel to fortite to mb.