Better test is to turn off Wi-Fi and cellular data, which will leave only NFC on with no internet connectivity.
I don't believe it needs an internet connection. The POS terminal communicates with the bank, not your phone. The phone just sends your Device Account Number and random security token to the terminal and that only uses NFC.
Airplane mode disables the NFC antenna. Better test is to turn off Wi-Fi and cellular data, which will leave only NFC on with no internet connectivity.
It works without internet connection... once your card is added and confirmed (requires internet obviously), its like having your physical card in the phone. No internet required.