Him already being someone who was comparable to kaguya,then ontop of that absorbs massive amps of her chakra and becomes the juubi jin along with various other power ups,he should be easily far stronger than kaguya,
This is in direct contradiction to Hagoromo's statement tho
i believe as a sage he was humble enough to tell that to have respect towards his mother,for me I'm sure it was said only to hype up kaguya for the final battle.
Hagoromo is a knowledgable character with no reason to lie Naruto asked if Kaguya was stronger than him and he said she was unrivalled pretty blantant "yes she is stronger than me"
Another thing if naruto and sasuke taking half of ghost hags power and then later training for some years became stronger,but after becoming hokage Naruto stopped training because of too much work,now here we have hogoromo who constantly trained his entire life created nishu and preached it,
We have no idea if Hagoromo trained his entire life ninshu is also fundamentally different from ninjutsu so even if he was training it wouldn't have been in the way you're thinking since ninshu is a pacifist ideology so he'd have no reason to get stronger with the greatest threat (to his knowledge) defeated.
mastered yin yang senjutsu,mastered his kekkei mora rinnegan(mark this is the strongest rinnegan),mastered all 5 natures mastered all 5 natures and mastered the treasured tools,as he trained his entire life and already being stronger than teen naruto and sasuke he should be clearly the strongest character bar maybe prime issike or Otutsuki king.
He might have already mastered most of these things by the time he fought Kaguya (aside from the Ninja Tools) we just don't know