The allegation is that KMnO₄ is undergoing a redox reaction with HCl to produce Cl₂ gas (which is a plausible redox reaction, given the half-reaction potentials).
But yeah, in practice you're going to disinfect with Cl₂ gas or NaClO. The water plant I worked at switched from Cl₂ to ClO¯ as primary disinfectant, and the operators were all for it. No more need to manually switch out Cl₂ tanks, no more Cl₂ venting issues (spilling NaClO is nowhere near as bad a Cl₂ leak). (The plant also used O₃ for bonus disinfection as well. The operators hated it at first, but now would throw a conniption fit were it broken).
Why not just add a hypochlorite to water if the end goal is to disinfect with chlorine?