No they are not the same thing. If I just turn off my cellular data, then that means typically that I don't want to go over my data limit and be charged more. This stops that so it means that only wifi will be used or I won't be able to get to the internet without wifi. I CAN STILL MAKE PHONE CALLS .... key point.
If I put the iphone into airplane mode and then turn on wifi after that, I can NOT make any phone calls incoming or outgoing. Yes I can still access the internet with wifi to use any apps or download apps, use camera, use anything else on the phone, etc.
This is why when traveling in another country most like to use airplane mode, it guarantees no phone calling but you can access the internet. You can also use facetime to talk to people in another country and also send and receive imessages because imessages use wifi or celltower, whichever is available, so in your case wifi is available. This means facetime and imessage are completely FREE, even if sending and receiving in other countries internationally.