I don't have access to the quoted 61558-2-6 standard, only 61558-1, anyway these standards are quite unreadable for the average EE. Dealing with the standards must be your day job to understand them, or go and ask a lawyer (or VDE person) of your least mistrust.
In general saying something is build according to a particular standard doesn't say too much, as there are often multiple classes or levels of whatever this standard shall achieve described within these standards. So for your question, it isn't safe to assume a transformer is "double insulated" when it is stated it is build to 61558-6-2 alone. If it says "safety class II" according to standard ..., it might be double insulated to achieve this, but there might be also other ways to achieve this class. Often this is quite difficult to find out by reading the standard, as these often written in kind of legalese.