Yes it is a big hurdle. However, I think content discovery is generally a big part of any content platform, way broader than discovering "who have reacted to my content". Now if you want to solve the problem of content discovery in a broader sense, then you have already fixed this particular shortcoming of pull-model as well. If a service that can inform you about new posts with a particular hashtag, it most probably can also tell you about reactions to a particular post.

And yes, I do realise that such services will tend to not be really decentralised (similar to the relationship of websites and search engines). But that means the downside is not that you don't get such discovery, but that you'll be reliant on more centralised services for such discovery, whereas in the fediverse you would be less reliant on such services for finding out who has commented on your post (though it will, as you've mentioned still not be enough).