Do they advise the same in the case of Triumph?
I only ask because of the different types of pumps used.
The Triumph pump, for all its shortcomings, wears well and develops good suction on its inlet even after many 10’s of thousands of miles use, so can easily handle a Charlies filter with cold oil.
I see no reason why a good BSA gear pump shouldn’t do the same (and we are talking about a new SRM pump).
Of course if the filter isn’t changed reasonably frequently, it will cause problems in either situation.
I think the argument that oil should be filtered on the way to the crank rather than returning from it is largely specious and mainly based on automotive wet-sump necessities.
If the oil in the tank is pure to start with, a return filter will keep it that way (other than discolouration), so the oil to the engine is clean.
As said, no harm in having filters in both lines, if you enjoy the expense and bother.
Something to brag about at the pub, when next allowed to go there!