[Spoilers] [Theory] Things are starting to make sense

[Mr Hiru]

Hashirama had a special sealing technique that could trap the Juubi. This technique was also used to stop Madara just before the Uchiha was revived.

Hashirama also was able to recognize the Rinne Tensei no jutsu.

Now we know that Hashirama/Madara were reincarnations of the sons of the Sage. It makes sense to say that they could recognize ancient knowledge.