As an argument, it barely passes initial scrutiny suggesting the real reason is not that.

The bundestags president, Julia Klöckner, was recently a victim of phishing on signal. While there could be different motives behind her suggestion, I think they are just another facet of her not really understanding technology and security practices.

She thinks she was "hacked" on signal, and now wants to switch to something which is clearly better! Let's wait where she will want to go once she gets "hacked" there too...

While there are valid reasons for germans not to want their politicians to use private messenger apps on their private phones for official business, and american ones at that, this switch would of course change nothing about all of these problems. But at least they can claim they did something, right?

My tin foil hat says it has something to do with chat control, i.e. being able to intercept messages or to siphon messages from Alice or Bob directly.