On googling this all I can find is various comments around the idea that because the oxidation state of P in H3P04 is +5 it can’t be oxidised further and so therefore can’t act as an oxidant.
That is wrong, its opposit. It must say it can't be used as an reductant.
If something works as an oxidant , it will be itself reduced, it gains electrons. This is theoretical possible with phosphorous. From oxidation state 5 it can be down to 3 or even to 0 or -3.
In opposit if something works as an reductant then it donate electrons. Itself it will be oxidised.