Does Reporting my phone lost immediately lock phone even if in airplane mode?

No, because the phone isn’t communicating with anything. As soon as Airplane mode is disabled it will be locked. As you (hopefully) have a passcode on your phone your content is still secure until the phone has an Internet connection.

Tracking in airplane mode uses the crowd-sourced Find My Network and the fact that BlueTooth Low Energy remains on. It depends on other people with iPhones passing near your lost device. It is not as useful or reliable as Find My when the phone has a network connection, and it only stays active for 24 hours.