I could be wrong but my understanding of the EU interoperability laws is that they cover interoperability between services (for example, being able to message a Telegram user from WhatsApp and vice versa), not interoperability with third-party clients apps.
With the new EU’s interconnection laws I hope I can WhatsApp from Pidgin, or even from irssi!
But no, I don’t use pidgin anymore. irssi, yes.
I could be wrong but my understanding of the EU interoperability laws is that they cover interoperability between services (for example, being able to message a Telegram user from WhatsApp and vice versa), not interoperability with third-party clients apps.