Note that the Colombia guide wasn't the only one Kohnstamm worked on
No, but it was the only one of which he said that he didn't even visit the country, which would've been far worse than accepting freebies if he'd contributed to any of the chapters on actual regions of the country.
He's just slanting things to appear to be more sensational than they really are - just like he probably slanted the descriptions of a few hostels and restaurants where he received freebies. And sure, it means you now have to take recommendations on places in "his" chapters with a large grain of salt - but the places themselves will still actually exist.
(That said, I'm very much going to be looking forward to the new edition of the Chile LP - comparing how recommendations there differ - before travelling to Chile myself. (For people heading there soon: I'd do the same comparison with a previous LP to which this guy didn't yet contribute.))