If you know what a derailleur is, odds are you know more about bikes than 90% of the customers.
The VAST MAJORITY of people who ride bikes don't know the first thing about them. The people here in this message board who ride every day and have old bike parts in every room of their house are not representational of the average weekend rider.
So not being a full expert may not be a deal killer.
This is for the typical local shop that caters to the entry level consumer. If it's a specialty shop that caters to high end shoppers looking for $4000 bikes, then the customers will know more than you do and they likely won't hire you.
You just need to be honest about what you do know and that you are a quick learner and willing to put in the work to learn.
Keep in mind it's a sales position. You are selling yourself at the interview. If you can sell yourself to a bike shop with the flaw of not knowing bikes, you can sell a bike to a customer.