I'm a bit confused about why he wants to steal these recipes from his master rather than use his skill, but if I recall correctly it might be because he doesn't want to use up a page from his skill with a potion/product he won't be able to implement due to lacking the ingredients.
I guess this finally explains the title; he's a fake because he has access to recipes and perfect implementation with none of the investigative ability, knowledge and experience developed by a real alchemist. With that said, while I understand these skills are important to develop, at some point the existing knowledge becomes too vast and in order to expand it you need to learn the conclusions.
Also even him having a backstory with this master of his feels like it's totally out of nowhere. So he got reincarnated here from Japan and began his adventure, went and got an apprenticeship and struggled to learn anything from his master, then passed the exam and got kicked out, and struggled to make a living on the streets until he picked up Nora and Coco? All of that while having cheat skills from the start? Confusing order of events. It's like author needed to jump straight to the harem building and figured he could backfill important plot exposition later.
Well, thanks to his cheat skill, he is a godly alchemist, but the issue is that he had no work permit as an alchemist and that his godly creations would have attracted attention and lead to trouble. That's why he had to start as an apprentice. I think it was mentioned before though we didn't have the details on his time with his master.
It's interesting that his alchemy master instructed him to observe and understand rather than just follow recipes with no understanding, because he's been doing a lot of observing this entire time, so clearly that time spent training wasn't wasted.