Of course we can explain it. None of that is relevant to the problematic positioning in the rule:
The only explanation given is part of a proficiencies list, in a parenthesis, with reference to equipment that is not present on the equipment list (non-metal shields).
As it is, every player and every table has to
(a) assume that wooden shields are available, have the same mechanical benefits as metal shields and cost the same
(b) make some sort of judgement call on armours
(c) make some sort of general-to-specific judgement call concerning racial proficiencies (e.g. dwarf)
(d) make some sort of general-to-specific judgement call concerning multi class proficiencies.
(e) reconcile the only actual behavioural restriction in 5e for PCs exists as part of one class's proficiency list
No one, I think, has a problem with (a)
Many people would have a problem with (b) -- there should just be a list of proficient armours.
(c) and (d) are not obvious; my instinct (that race and multi class are the "specific" cases, representing a subset of druids who behave differently) will not be that of everyone
(e) demonstrates that it's just bad writing.
Being able to imagine a reason why this might be so is irrelevant. Not spelling out the actual implications of the rule (how it affects druid magic and abilities, etc.) is what matters at the table. Since this will affect anyone playing a druid, ever, it should be clearer than this.