What is the reference being made by saying "I did this at a serious seminar once"?
The 進研ゼミ (Shinkenzemi) is a one of the most famous distance learning service for minors in Japan.

'真剣' and '進研', both read as 'shinken.'

'真剣' meaning "seriousness"

'ゼミ' is short for "seminar."

'進' means "progress/advancement".

'研' means "sharpen/polish/study."

This is a parody that use similar sounding Japanese characters.


  あ、これ進研ゼミでやったことがある 
  
  I did this at the Shinkenzemi once.


Much like McDonald's and MacRonald's in English.