This seems to he based on a lot of hypothetical and not actual data.

The study examined the carbon footprint associated with each user per minute by incorporating the emissions associated with data centers, which made up about 99% of the footprint, and the emissions associated with charging devices after using the platforms.

TikTok’s emissions are the most opaque of the social media platforms. Tech giants such as Meta and Google release detailed reports to the Carbon Disclosure Project every year, even posting their findings to their respective websites. TikTok has no publicly available emissions data.

It's just the theoretical output of emissions needed to run a Data Centre based off viewers and average time spent. While these are all rough numbers it could also very well be that the Data Centre's are powered at least in part by renewables.

So it would make sense that Tiktok would use up a lot of electricity for its platform. We just can't be sure how much of it actually translates into more emissions.