As mentioned by @W.Are in the comments, nobody witnessed Konohamaru perform the Transformation Justu. With enough skill, the transformation turns the user into a perfect replica.
The transformation of a skilled shinobi will be exactly like the genuine article, so it will be impossible to tell the two apart
(emphasis my own)
While shinobi who have the ability to detect chakra signatures (such as sensory types) could have easily detected this had they been actively looking for it, this was an inauguration ceremony during a time of peace. The thought of an imposter was probably not on their minds.