'Kecap' is just soy sauce. It is not used typically as a stand-alone word for sweet soy sauce; sweet soy sauce is kecap manis, while kecap asin is salty soy sauce.
Yes, there are similar roots from hundreds of years ago. Mental Floss did a good article about the two words!
Wait, if 'kecap' is 'sweet soy sauce' and 'manis' is 'sweet' then 'kecap manis' is... hyper-sweet soy sauce?
I'd say much like "kacang" means nuts in general but unqualified generally refers to peanuts, it is the same for kecap manis, since it's the most popular one. I think it's correct that when people just for kecap they usually mean sweet soy sauce