You're totally right to say that in the context of TZSET, to load the timezone specification from a TZ-formatted file it needs to be prefixed with ':'.

Rather than saying "Technically, you should set the value of TZ to 'Europe/London'", I was trying to say that my philosophical opinion of timezone recording - whether in a CMS, on an O/S level, in a compiled app, etc - is that it should start with the standard of geographical location.

There might be occasions to augment that with other data, maybe including the UTC offset for that TZ at a particular time, but the TZ specification can be subject to frequent change, whilst a description such as "Europe/London" changes infrequently and seems to have the least ambiguity.