Unsure if I can use thin crib mattress?

I purchased the DaVinci Kalani mini crib and it comes with a very thin maybe one-inch “mattress”. I went into my local baby store to buy the 5 inch DaVinci TotalCoil mattress to replace the super thin one for my newborn baby but the owner told me it’s actually recommended for newborns up to 6 months to sleep on that thin mattress that the crib comes with and that the thicker mattress is for when they get older. Is this true? I didn’t find that info online when I was researching and just want to use something that’s going to be safe as well comfortable for my newborn.