The images in the BBC article suggest that the intention is to utilise on-street trams through the development. But then as someone already mentioned, the Metro is all just talk at the moment. Where is the real meat of the Metro proposals? Some fake images with no real logic won't cut it.
I think the pictures of trams reflect the fact that a few of the main spine routes are being made wide enough so that they could carry trams that are partially segregated. The S106/infrastructure levy (or whatever its called now in Wales!) won't be anywhere near high enough to pay for such a tram system into this development though - although it would be a not insignificant contribution to a bigger pool of funds for such a scheme.
I think the best bet for this development would be semi-segregated buses though. Much cheaper and much more flexible than trams. Of course, there would be bottle-necks on leaving the development. To reduce the impact of this, I would have a bus-only route connecting the southern part of the development with St Fagans road. I'd also make improvements to the Waungron Road junction so buses can more easily turn south/west along Western Avenue if that route into town were less congested than continuing down Waungron/Pencisely Road.
In the long-term I'd put a route into the A4232, and have express buses come in that way.