HEPA filters won’t stop COVID going from person to adjacent person. No one has ever seriouslyclaimed that.
What they do stop is COVID recirculating throughout the cabin.
One thing that Airbus’s cabin airflow studies recommend - and I have yet to see on 4 airlines I have paxed on in the last few months (including QF) - is closing the overhead air vents prior to pax boarding. Open they will both 1. Spread particles further and 2. Suck any particles into the airflow down onto the pax.