That is true, but there is a good reason. For example, you may call 911 in North America without any cell plan, or without a SIM. As you long as you are within physical range of any cell tower (whether your phone shows bars or not) the 911 call will go through. This is required by law. So, like your quoted text indicates, 911 calls would just need to be routed through your phone’s native dialer instead of, let’s say, Cheogram’s dialer (jmp.chat’s phone/message app).

What security risks are you considering for physical SIM?

You can keep your cell number with jmp.chat. Call over wifi or data. They offer eSIM. View text messages on any device/program with XMPP support. 2FA works 100% like normal unlike VoIP. All data, calls, texts are routed through their VPN first, then the cell network. Any other inhouse XMPP chat not going to networks stay within XMPP. I have no affiliation with jmp.chat, I am satisfied with the service.