It’s all just a big “in theory” really. It’s “insecure” in that if someone knows the telco you are with, and the telco that you’re with doesn’t follow procedures to verify that a caller is who they say they are, you could have someone else steal your phone number by getting a replacement sim card sent to them.

In reality it’s nothing to worry about. Like…at all. Every telco I’ve been with sends you a sms to confirm that you requested a new SIM card, and that’s after they’ve confirmed that you are who you say you are via sending you a code on your phone number or email.