The full quote is:
"Dr Harries told BBC News: “What tends to happen is people will have one mask. They won’t wear it all the time, they will take it off when they get home, they will put it down on a surface they haven’t cleaned.Or they will be out and they haven’t washed their hands, they will have a cup of coffee somewhere, they half hook it off, they wipe something over it. In fact, you can actually trap the virus in the mask and start breathing it in".
Asked if people are putting themselves more at risk by wearing masks, Dr Harries added: “Because of these behavioural issues, people can adversely put themselves at more risk than less.”
So not about mask wearing
per-se, but poor hygiene and lack of care being an issue. In the same article, Dr Jake Dunning said “Face masks must be worn correctly, changed frequently, removed properly, disposed of safely and used in combination with good universal hygiene behaviour in order for them to be effective.”
What's changed since March is that we know more about the virus and more research has been carried out into how it spreads. I linked to some papers on it earlier. A study in May by
Johannes Gutenberg University Mainz and the University of Southern Denmark found that mask-wearing could cut transmission by 40%. The evidence went from weak and unclear to marginal but positive.
Harries also said that large gatherings were not a source of infection and there were calls for her to resign over statements about testing too.