I'm not a fan of Obama, but I'm going to give him a pass here.
it's well established that both Russia and China continuously attempt to hack into everything: the white house, the pentagon, the state department, defense contractors, university research labs, the electric grid, the water system, various state and national level political databases.
revealing "how we know" is likely only to compromise the tools/methods we use to discover and deter such attacks.
A more important question - two actually.
1 - Obama - what actions did you take during the past 8 years of incessant hacking to protect our security?
2 - Everyone - what difference does it make who did the hacking, if the DNC emails were 100% factual? An election can't be "stolen" with facts - only with lies.