That sounds like some rubbish a salesman would say, because it's not accurate.
Any radios purchased with federal grant funds must be P25 capable* and meet P25 CAP (Compliance Assessment Program) standards. That's the rule you're probably thinking of, however it most certainly does not apply to every public safety radio.
There is no law that prohibits a public safety agency from purchasing a MOTOTRBO system and subscribers, for example, which are not capable of P25 (i.e. the XPR line, which is analog and DMR/TRBO only). They can't purchase that equipment using federal funds, but they are perfectly within their rights to buy and use whatever the like with non-federal funding.
* The radios merely have to be capable of P25 either out-of-the-box or by way of future upgrade; they do not have to have P25 enabled/provisioned at the time of purchase.