Bullshit, we had an iPhone 6, only seeing an O2 (UK) card for a year. After that it just accepted a Hungarian card (Telenor HU) without any issues.
But beware with one thing: "unlocked" from Carphone Warehouse in UK means "unlocked for any UK networks", not internationally (that's why it is cheaper). Buy from apple.co.uk if you really want it to be surely unlimitedly unlocked, nowhere else. They are saying it on their own website when you try to order a "sim-free" one:
"iPhone will lock itself to the network of the first SIM card that is used in the phone. Subsequently attempting to use it on any other network, for example by replacing the SIM card, may mean it becomes permanently unusable. SIM free iPhones can only be used with a UK-network SIM card, and will not accept foreign SIM cards.".
But again: this is only Carphone Warehouse UK (and maybe something else I don't know of).