This hasn't been errataed yet or explained by the designers, so the reason for the discrepancy is unclear.
As you note, #10 under the "Side Quests" section (ToA, p. 17) has an NPC tell the party to find someone at Orolunga, "east of Mbala" - even though the map of Chult (p. 39) shows Orolunga as west of Mbala. However, no explanation has been given for this discrepancy, either in the adventure or in a designer statement.
Tomb of Annihilation was updated in June 2020 (a month after this question was asked), as of the 6th printing of the adventure, to remove some offensive and correct a few minor errors. Only some of these changes were noted in the errata PDF for ToA that was released later that year. However, this apparent error in side quest #10 was not one of those corrections.
As such, it's impossible to know for sure what the reason for this difference is unless it is fixed in a future errata (which would indicate that the NPC was not meant to mislead the party, and that it was just a typo) or unless a designer makes a statement addressing/explaining the issue.
A number of users have wondered exactly the same thing as you in this 2018 /r/dndnext thread and this 2020 /r/TombofAnnihilation thread, but there doesn't seem to be an overall consensus on why the NPC wrongly describes the location of Orolunga (though "typo" seems more likely).
A 2017 D&D Beyond forum post also notes that both the DDB version and the physical version of the adventure contain this inaccurate phrasing, and asks if DDB can fix it; a DDB mod responded that DDB has to replicate the physical version of the adventure, which means that the possible typo won't be fixed unless/until Wizards of the Coast makes that change in the physical book as well.