That is just testing if COVID19 breaks down when exposed to mouth wash. For that to prevent infection you'd need contact between the mouthwash and the virus.
So maybe mouth wash helps or maybe it's useless because people are getting infected in their nose or before they use mouthwash. Pretty hard to say but this study doesn't tell you anything actionable.
Unless you're in Australia or New Zealand, how would you even have caught a cold? Even with places partially reopening, it seems more likely it was allergies.
That is just testing if COVID19 breaks down when exposed to mouth wash. For that to prevent infection you'd need contact between the mouthwash and the virus.
So maybe mouth wash helps or maybe it's useless because people are getting infected in their nose or before they use mouthwash. Pretty hard to say but this study doesn't tell you anything actionable.