Who knows about the vehicle better?
1. A*to Mi*** ?
2. KIA Service center (in SL)?
3. Manufacture (who prepare the service manual)?
If 1 or 2 are manufacture of KIA, then you should follow them. No mate, those people are bullshit. They only bother about how much could they earn from you.
Even with Colombo traffic, we (Sri Lanka) are well below the extreme driving conditions. Following the owner/service manual is the best practice. 40k for ATF is now obsolete for most of the vehicles. However normally the manual says xxxxxx km or xx months whichever comes first. So please refer not only the millage but also the duration mentioned in the manual. You have to replace fluids in correct duration irrespective of lower millage.