I would regard a figure of around 750k discouraged voters as much more plausible - still a serious problem which needs to be addressed. I suspect that the most part of the decline in voter turnout between #ukgeneralelection2019 and #ukgeneralelection2024 was due to other factors.
This looks like the high end of the range of estimates for the effect. If accurate it would account for over half the reduction in voter turnout observed between #ukgeneralelection2019 and #ukgeneralelection2014 .