Whilst かっこう (the noun) can refer to 'manner' it also refers significantly to 'shape' or 'form' or 'appearance' which are quite shallow and visual things to perceive.
And besides, in English, saying that someone was or looked cool are in many cases functionally equivalent sentences. For example, there is little difference between:
"I went to see [singer] live on stage - he loooked really cool!"
and
"I went to see [singer] live in stage - he was really cool!"
As for where the 'really' comes from then well, probably the exclaimation mark? Just imagine it said in an excited schoolgirl voice and you aren't far off.
Whilst かっこう (the noun) can
Whilst かっこう (the noun) can refer to 'manner' it also refers significantly to 'shape' or 'form' or 'appearance' which are quite shallow and visual things to perceive.
And besides, in English, saying that someone was or looked cool are in many cases functionally equivalent sentences. For example, there is little difference between:
"I went to see [singer] live on stage - he loooked really cool!"
and
"I went to see [singer] live in stage - he was really cool!"
As for where the 'really' comes from then well, probably the exclaimation mark? Just imagine it said in an excited schoolgirl voice and you aren't far off.