Hi Questionper,
The article Erase a device in Find Devices on iCloud.com offers the following regarding erasing a device using Find My:
"What happens when you erase a device in Find Devices?
- A confirmation email is sent to your Apple ID email address.
- Activation Lock remains on to protect it. Your Apple ID and password are required to reactivate the device.
- If you erase a device that has iOS 15, iPadOS 15 or later installed, you can use Find Devices to locate or play a sound on the device. Otherwise, you will not be able to locate or play a sound on it. You may still be able to locate your Mac or Apple Watch if it is near a previously used Wi-Fi network."
as well as:
"If your device is online, the remote erase begins after you follow the onscreen instructions. If your device is offline, the remote erase begins the next time it is online."
The expectation would be that if the device is offline presently and unable to communicate with Apple servers, the request would not be received and processed. You cannot turn a device on in airplane mode but instead have to change that setting once turned on. So what happens would depend on if a connection is established and that command is received before airplane mode could be enabled at startup.
Note also that unless you had iOS 15 installed on the device, once erased you will not be able to find it using Find My any longer if that command is processed.
Regards.