Unless they changed the terms of what is posted on the website, you will still have to activate the T-Mobile device through T-Mobile, which means you have to pay for at least a 1-month plan:
If you have selected an iPhone for T-Mobile, it will come with the appropriate nano-SIM, but you will need to contact T-Mobile or visit an Apple Retail Store to activate your iPhone.
When an actual unlocked version of the device is released (one that is not associated with any carrier), then you will be able to do the following:
If you have selected an unlocked GSM phone, you can insert your own nano-SIM card or obtain one from the supported GSM carrier of your choice and activate it in order to use your iPhone.
That is in writing, on the Apple iPhone 6 site. So, either the information has changed and the site has not been updated, or you were misinformed.
Cheers,
GB