I'm by far not a "metadata specialist", but I know a little bit about files. And I can tell you that anything from a file can be changed, including the metadata.
If the file would be protected by a checksum or anything, then the checksum can be edited too, but here it's not the case.

I've also done a quick experiment. I've opened the file as text in Notepad++, edited line 15 (creationdate) and changed it to 2001. And saved it. My pdf viewer (FoxitReader) opened just fine the modified file and now the creation date is in 2001.

Conclusion: you cannot trust that and in fact you are chasing for something that cannot be tested from the metadata to be truly genuine.