Also, by doing that and letting the account existence be publicly known, anyone with their phone number will know that they have an account on that service and lets you try to contact them. That can range from privacy violation to life-in-danger situations.
We give too much information away. Companies have shown that they are incapable of keeping the information private anyway. We get big scandals, the interwebs kick up a big stink, brows are furrowed, and then we go back to doing what we did before with nobody learning anything.
If I gave my phone number to someone, I made an implicit social contract with that person or organization that they may contact me using that phone number. I don't see it as a big stretch if they use that number to send me a message or call me using WhatsApp, Signal or any other messenger where I registered using that same number. It's for the same purpose of communication, after all.
If I don't want to be contacted, I just don't share that number.
And it's not like people don't know that messengers or social networks can collect contact metadata about messages and calls. It's what traditional landline and mobile providers have been doing for ages.
I don't really care if they sell my phone number, it's already out there, I don't consider it private data anymore.
It’s basically an email client disguised as a messenger.