Why is mining described as validation of a block through proof of work when it is the validation that is critical not the proof of work? That is reward for proof of work provides incentives for miners to validate transactions; it does not validate them through proof of work.
You are correct in that the proof of work itself does not validate transactions, that is done separately, so that only valid transactions will be included in the block. 

However, the block itself must contain a valid proof of work (the block hash). Without this proof of work, the block would not be valid (even if the transactions are valid). So it can be said that a block is made valid by its proof of work. 

Edit: it is worth noting the distinction between the act of performing the proof of work hashes, and the object that is the proof of work (block hash). My answer references the object, not the act.