Solution to design a schema without joins

I read in the blog you shouldnt use Joins in your queries.

I am really not getting it, in my case.
I have data like in the common example schema, where in the review table is a column reviewer.
By time i collect customerids and i use them to mapp to transactions. Right from the beginning this mapping is incomplete.

So whenever i update data in one of the related tables i get maybe a new match (succesful join).
Since one customer can purchases multiple times and write a review whenever he wants, maybe even before he orders.

when i am wrong here with this type of question, i am gonna delete it.
Thank you so much i really like Metabase so far