Depends on what you mean with 'reliable'.
I'm sure the numbers are correct. But they only track what languages people talk about.
While popularity is one reason for people to talk about a programming language, there are many others.
Newer languages will get more traffic than older ones since people have more questions about them.
More complicated languages will get more traffic because people have more problems with them.
Also, the granularity sometimes seems off. Assembler is a good example: there really isn't a single assembler language; there are many different ones. How useful is it to lump them all together?
So this index does provide some interesting data, but you have to be careful when trying to draw any conclusions.