This is not correct. Scanning a QR code into the TousAntiCovid app, or indeed any other app, does not change it in any way.
The app may not display an expiry date, nor may it show any error after the expiry date passes. It may even continue to work in France, if their validation app (like Ireland's) is programmed to ignore expiry dates.
The underlying QR code will still contain the expiry date, and it will not work with a validation app that checks the expiry date, such as any of the UK ones.
I've since checked this, and the English covid status checker app was displaying an expiry date of roughly a year since I scanned my Scottish acquired vaccine QR code into TousAntiCovid, which seems a signifiant upgrade on the expiry dates offered by the various NHS apps.