Just wondering if I am correct in saying that if an empty enclosure (prior to the incorporation of switchgear and control gear components by the user) is provided by an "enclosure manufacturer" and is compliant with IEC 62208, to an "assembly manufacturer (enclosure with switch/control gear installed), there is no need for the "assembly manufacturer" to carry out Strength of Materials and Parts verification as per IEC 61439 ?

 

You appear to be overthinking a crossover point in Legislation whlist pointing into a grey area.

Having seen manufacturers make both enclosure and switchgear installation, compliance is no problem.
Where enclosures are bought in, compliance to IEC 62208 is at point of purchase. Fitting out could affect that but generally doesn't.

A late 1990's employer bought in a few Nema4 rated MCC boards. Despite cutting out doors for pushbutton, meters and the odd HMI, the finished product wasn't and wouldn't be de-rated.
In 1995, a very hot summer in the UK, they had thermal problems which turned out to be the switchgear, creating the need for forced ventilation to be added.