And there's some additional detail here:
https://cardiff.moderngov.co.uk/mgAi.as ... gDocuments(In the one document that isn't restricted from public view). It specifically mentions a footbridge link to the Pierhead Street car park. This would cross the dual carriageway just east of the tunnel portal, so I hope they come up with an elegant and perhaps interesting design.
There is planned to be a public exhibition of the proposals later in the summer, subject to whatever the rules about you-know-what turn out to be.