SIM free does not mean that the phone is not locked to a network, so the statement makes perfect sense and clarifies the situation.
SIM free means that the handset is being sold without a SIM and you need to provide your own to use it. Not locked to a network means that the handset isn't coded to only connect to one network.
They often come together, but they are not the same.
I'm behind the times. I only use my mobile once in a blue moon to make the odd call but I gather that the newer versions are like a mini computer, hence my confusion.
If you want calls and text only Co-op Mobile are among the cheapest - free sim from stores or online (uses EE)
If you also want data look at giffgaff - they do 'goodybag' bundles without contract (use O2) - free sim online.