The TLDR of the paywalled article:

The order stems from fears that the chat group may have become the target of targeted cyberattacks.

The decision was made according to the report after the Commission became aware of the group’s existence last month and deemed the risk of compromise too high. While there is no evidence yet that communication has actually been intercepted, the threat situation has escalated.

The Commission is now reacting with stricter IT guidelines and regular checks of employee hardware.

Recently, Dutch authorities warned of a global campaign in which Russian cybercriminals are using fake Signal support bots to lure users into traps.

I feel the title is really not in line with the article contents: the only thing it says about signal specifically it is that it lacks some security and management features common in state run infrastructure.

It seems it is more a case of generally tightening the rules about the politicians’ communications’ channels in general.