You can find the Council's Tall Buildings Supplementary Planning Guidance on this page and there's no mention of such a policy.

https://www.cardiff.gov.uk/ENG/resident/Planning/Planning-Policy/Supplementary-Planning-Guidance/Pages/Supplementary-Planning-Guidance.aspx

If there is such a policy as mentioned in the opening post, I've never heard of it.

Well, there you go. If Barden hasn't heard of it, it can't be true!

I remember hearing the same thing many years ago i think late 90's around the turn of the millennium. Maybe it was a policy never implemented or from further in the past.