In my experience this is not true at all, especially in huge groups. I was participating in a co-housing project and after a few weeks the obligatory WhatsApp Group was forming. Immediately some people refused to join and proposed Signal / Threema instead. End of the story, the WhatsApp Group is still alive and the biggest of them all. If you are not part of it you will miss important information. Sometimes it gets forwarded to Signal but it is a rare occurrence.

It only works reliable for messages directly addressed to a person. Then people remember, oh he does not have WhatsApp. I will tell him via signal.