You can see how that the lack of details is basically asking me to… trust you without evidence?
Edit: to use less ’gotcha phrasing, anonymously claiming that another organization is doing better on feedback, but not telling me how, is asking for me to blindly trust you for very little reason.
I don’t think feedback practices are widely considered secrets that have to be protected, and if your familiarity is with the UK Civil Service, that’s a massive organization where you can easily give a description without unduly narrowing yourself down.
I instead read this point as saying “assume that if we persuaded 100 folks to give up carp, then 1 of those would replace their carp consumption with salmon.” So it’s talking about the replacement effect, rather than the number persuaded (the latter gives magnitude, as you say).