It’s called certificate-pinning if they “hard coded” a certificate, yes then you have to replace/remove it and move the same certificate to your MITM proxy to decrypt traffic.
Note for other readers: the two sentences in the parent post have nothing to do with each other. The Robert Heathon post does not describe how to break cert pinning without needing hardware/firmware modifications unfortunately.
The only relevant thing to repeat here might be this
> if you can’t get access to your IOT device’s hardware in order to add a new root CA to it, your journey mostly ends here. Don’t lose hope though. Leave your setup running for a while and see if anything strange happens. When I attempted the above process on my baby’s crib, the device refused to trust Burp’s certificate and so refused to complete a TLS handshake with my laptop. But after a minute or two of repeated failures, the crib started sending out some of its system status data over plain, unencrypted HTTP!
A good write up here for ways to try to intercept IoT devices without any hardware/firmware job: https://robertheaton.com/2019/11/21/how-to-man-in-the-middle...