You can use the Tahuk Festival ability to destroy pops. It's available on the city management screen.

Indeed, the Tahuk festival is available in every city. Except that it will prefer the summoned rather than the Tahuks, which doesn't help get rid of them.

Dude, if you look closely, you'll find evidence that the game isn't tailored to any set pattern at all. For example, during the starting faction selection, if you hover over the festival, it says it's only available during monsoons. But since that would ruin the main questline, they obviously decided it would be easier to make them available continuously, thereby violating the lore. And this is just one example, of which there are many if you start paying attention.
And the answer to your question: the is no way to finish that achievement at this point, at least in normal play

This leaves me with the feeling that we simply don't have access to some of the game's features, ultimately.
What's frustrating is that I'm not sure.

However, in another section, there's a guide on how to complete this challenge, so I guess it is possible? I presume so.

I just went for this achievement and actually got to do it. Problem is, the achievement seems to be bugged :D

1, play only with 1 city
2, go for destroy 10 fortresses victory
3, kill your pop continuously during game with festivals
4, time it so that the turn after you kill 10th fortress, you activate festival, that kills all you pop in your city
5, activate 1st ability to get 1 called pop

I literally have only 1 called pop in my empire and the achievement is not triggering.

Well, same thing, I managed to win a game by decimating my Tahuks throughout.

At the end of the game, I find myself stuck with 1 or 2 called, and I manage to start the last turn in this condition (by defeating my only opponent).
I am “lucky” to have a plague in the sector that I can feed as a permanent sacrifice.

And of course, nothing happens!
Another problem is that my city, as small as it is, generates at least 1 inhabitant (Tahuk) per turn, which can interfere with the last turn... I haven't been able to get rid of it.

Number of hours spent on the challenge: 10
Success: 0

Update: I finally succeeded in this challenge.

It was more complicated than I thought, but I made one last attempt before giving up.

A very easy opponent, small map, few resources, no monsoon curiosity, lots of ridges/mountains.

After establishing my capital in an area with strategically few resources (0 food production), I built as many observatories as possible on the territories (except on my capital, which provides food...).
I managed to contain Tahuk production by triggering festivals as much as possible to keep the Tahuks between 0 and 2. I made sure they weren't in the “Citizens” box to limit food production. After a while (except during the monsoon season, which was a difficult period), food production became negative and famine set in. Tahuk production came to a complete halt: I triggered Eternal Sun and the other spell on each of my available observatories whenever I could.

I won the game with 4 called people in my population (and the Tahuks could no longer pop due to famine and negative food production).

Success was achieved!