Multiple people who have seen the entire Snowden dump claim that various files leaked by specific sources were not contained within the Snowden dump.

Yes, the idea that the "second source" and TSB are the one and the same is necessarily based on conjecture. Nobody is presenting it as a fact, but as a rather likely option based on analysis of data released by TSB and NSA leaks which cannot be attributed to Snowden.

Both TSB leaks and "second source" leaks originate from the same time period, and the same locations within the NSA. That does not mean that they were leaked by the same person(s), but it is a fairly likely option.