Read it again. The DC EMA only states “The line of protesters spread across Pennsylvania Avenue for blocks, all the way to the capitol, according to the Washington Homeland Security and Emergency Management Agency.”
They make no claim as to the size of the crowd. That is a claim of the UK reporter, who has no knowledge of DC crowd estimates. Their report comes from watching city-wide TV cameras that are turned on whenever a significant event occurs in the Capital. (Privacy complaints restrict the use of cameras at other times.)
Okok I give! ;-) I believe it was over 1 million :) :)