The moment the author used the birthday paradox in a wrong way to try and prove some point with the ID's, I stopped reading.

The points before that:

Being forced to use a unique mobile phone number is a major issue for many (me included). As is being based in the US.

Threema didn't yet have Perfect Forward Secrecy back in 2021, when this article was published, but it has now.

Yes, Threema isn't perfect. But neither is Signal.
Choosing which one you should prefer is up to your specific scenario.
Use a recent and trustworthy comparison chart, like e.g. the one by Kuketz.