Paul Farquhar
Legend
When the PHB was written, there where no things that where not spells that you could concentrate on.The question was if you can concentrate on things that are not spells anyway.
So the sense is "for those of you who happen to be multiclassed as spellcasters, this rule will only apply to you...". This is, of course, no longer the case, but the intent of the rule is you can't concentrate whilst raging full stop, not that being a spellcaster makes it harder to concentrate.