No, I think it is because 'population' is a whole thing, rather than a group. Not sure why, but I am a native English so I'm pretty sure I'm correct. Wish I could help you more.
I guess it's a British English vs American English thing. In British English, population is a plural word (because calling something a population when there's only one person seems pretty unintuitive)
Most of the time, this site caters more towards American English, because I guess there's more userbase that speaks it.
The normal rule - at least in England - is that if all the individuals who make up the collective noun act together you would use the singular but if the were all acting as individuals you would use the plural. So in this case if the whole population had embarked on a single training programme it would be reasonable to use the singular but if they had all gone away to do their individual things it would be more normal to use the plural. However different authorities have varying degrees of attachment to the use of the singular (although I can find none recommending it for the Police (eg the police is chasing a suspect). Oddly Duo - who often accepts some very sloppy colloquialisms - appears to be at the extreme pedantic end of the scale on this one! Whilst in this case although the plural sounds more natural I do not believe the singular to be wrong so I think both should be accepted.