That’s not a big difference (10%) between heavy red and lightweight grey squirrels. Also, that “typically” indicates the difference maybe smaller, or ranges may even overlap.
Deployment of any device targeting wild animals like this needs approval. Processes are stringent, programs for sterilisation and release of cats took ages to get approved. Rewilding is held up for years on assessment.
This is the least likely problem with this approach. Much more likely is that it isn't sufficiently effective and surprising things happen when the grey squirrel population is suppressed, which are counter productive.
I hope it works.
I'm not saying there's zero risk of a hormone disruptor in the food chain, but I don't think scots hawks and eagles predate squirrels as their main or sole food source: https://www.wildlifeonline.me.uk/animals/article/squirrel-pr... suggests its marginal from a squirrel population dynamic point of view.