Usually when people refer to their SDK in this context, they usually have an iOS and/or and Android SDK. These can only be used by Corona Enterprise as they are native object based.
Now SDK is a generic term. And it’s possible a service could build a Lua based SDK that you could use with Starter, Basic or Pro. There are SDK’s that are all based around networking API calls, which any Corona based app could use.
Now I did a quick lookup of layer.com and they are in the first category. They have iOS and Android SDK which means you would need Enterprise to use it. They do indicate that a web based version is coming. Corona Starter, Basic and Pro could use that.