from what i saw from manga (not read the LN yet), this is not isekai
yes, the MC is reincarnating, but still in the same world, no modern world knowledge
there's many akward moment here, like how the adventurers there believe so easily that a new 15 years boy killing a dragon by himself, or just no one envy him or scorn him for it, it's like the whole city is already aware that "oh he is the MC"
in chp 14, when the friend of MC surprised by lost magic performed by MC, the people around them is just, walking around like completely normal, no one is surprised beside the one who know the MC, even though the MC perform it in the middle of city
this little detail in manga maybe some added by mangaka, not told in LN, that make me annoyed to see it