He's probably talking about how on iPhones[1] and some Androids[2], you can do something called "wifi calling using cellular data" or "backup calling", which basically enables you to roam for "free" on your one SIM by using wifi-calling over the data connection of the second SIM. It only triggers if the first SIM doesn't have any reception, but there's workarounds for that, but in any case it's not as simple as installing an eSIM and getting free roaming.
[1] https://cdsassets.apple.com/live/7WUAS350/images/ios/ios-18-...