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I think they use @anyx's API. Luckily we don't rely completely on Steemit anymore.

i am not getting the feed function to work on any steem API :/ try your self in terminal with the command

curl -s --data '{"jsonrpc":"2.0", "method":"follow_api.get_feed", "params":["maxsieg",1,10], "id":1}' https://anyx.io

curl -s --data '{"jsonrpc":"2.0", "method":"condenser_api.get_feed", "params":["maxsieg",1,10], "id":1}' https://anyx.io

while it works well for the get_blog function

curl -s --data '{"jsonrpc":"2.0", "method":"condenser_api.get_blog", "params":["maxsieg",0,1], "id":1}' https://steemd.minnowsupportproject.org/

Sorry, I know barely more about this than @justinsunsteemit. Try asking an app developer or witness.

I just looked at the proposals, and it looks like @steempeak partly uses their own infrastructure.