No, the MATLAB Runtime is not free. It is a licensed product.
Versions of it for the several years can be downloaded at no cost by anyone with a MATLAB Central account, but it is still a licensed product.
For licensing reasons (MATLAB was leasing technology from other companies), sufficiently old versions of MATLAB Runtime cannot be downloaded in such ways, and are only available to people who have directly purchased MATLAB Compiler product, with anyone else being required to obtain the runtime from the person who build the executable. In turn, the people who built and distributed executables were entitled to distribute the Runtime for no cost, but they were expressly prohibited from making the executable publicly available -- for example they could make it available for FTP but not for public FTP, only for people who had specifically contacted them to request access.
Is it No Cost? Yes.
Is it Public Domain? NO . (The portion of software that is Public Domain is pretty small.
Can your company embed the Runtime within an executable built with your company's copy of MATLAB Compiler, and then make the resulting executable pubicly available? Yes.
Can your company make the Runtime publicly available by itself, outside of an executable you have built? NO .
For example, if hypothetically your company's business is creating software archives (for example such as MajorGeeks does), then NO you would not be able to archive the Runtime for public access, not even for no cost.