I noticed that both Proto-Sinaitic and Proto-Indo-European have the title of "proto", although the Proto-Sinaitic has actual scripts which were found and studied, i.e. it is a fact that it existed, while PIE is completely reconstructed. If so what does "proto" mean? Should Proto-Sinaitic not be considered proto anymore, but rather "ancient", or should it have it's own word?
By convention, the prefix proto- is only assigned to reconstructed languages predating the extant records of related languages. In rare cases it happens that older records show up and a proto-language gets to the status of a real language (the only example that comes to my mind is the case of Old Norse ⁒ Proto-Germanic).
However, Proto-Sinaitic is not a language but a writing system. The terminology of paleographers differs from the terminology of linguists in this respect, it allows the prefix proto for an attested writing system that is the predecessor of another writing system.