kanji makes life easier every kanji has a meaning of it's own more than just a sound like hiragana and katakana to make things more clearer check out this video:
grammar is very important so you can fully understand the whole sentence let me give an example:
若いうちに本を沢山読みなさい
the kanji:
若い=young
本=book
沢山=a lot
読みなさい= to read(an order)
so now we know all the kanji but wait a second we don't know what うちにmeans right? we can't then fully understand the sentence
we know that this means 本を沢山読みなさい which is "read a lot of books"
若いうちに means "while you're young"
the sentence is read a lot of books while you're young that's what うちに stands for "while" that's why grammar is important even though we knew every kanji there were still something we didn't understand
i don't know if that's what those people meant by they had to study it all over again ask them instead as i told you learn the basic kanji with grammar for now