It is very plainly and obviously not a "similar switchgear assembly" to a type-tested DBO as it contains no overcurrent protection. It is simply a multipole switch.
I suspect that a significant proportion of (the few) fires that originate in CUs arise because of poor connections to the main switch (after all, that's the place in the CU where the highest potential currents flow). I therefore imagine that the same hazard would exist in a standalone isolator, so one might imagine that those who wrote the 'non-combustible' reg (and the LFB who seemingly forced it upon us) probably will have wanted such things as standalone isolators to also be covered by the reg.
However, as far as I am aware, it is not possible to have a standalone isolator "to BS EN 61439-3" - in which case such a device probably could not satisfy the reg even if there were not the additional 'non-combustible' requirement.
"Similar switchgear assembly" is such a vague and imprecise phrase that it really should not appear in any Standard, legislation or official document. As you sort-of imply, I wonder what, if anything, would qualify as being 'similar to' to type-tested DBO - I certainly can't think of anything particularly 'similar'!
It can only be presented as unclear or definitely not including an assembly by people ideologically driven to want it to be unclear or definitely not including an assembly.
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