Why would a stretch variant need a larger horizontal stabilizer?
When the fuselage is stretched, the arm of the horizontal stabiliser is increased, and hence it's effectiveness increases linearly with the fuselage length. 

However, because the mass is distributed further away from the centre of gravity, the pitch moment of inertia increases too. 

If the fuselage would be modelled as a uniform rod, the moment of inertia in pitch would be $\frac{1}{12}mL^2$, with mass $m$ and length $L$. 



source: hyperphysics.phy-astr.gsu.edu 

You see that the moment of inertia increase with the square of the fuselage length, while the effectiveness of the elevator increase linearly with fuselage length. The nett result is that the pitch response of a longer fuselage with the same horizontal stabiliser/elevator is reduced. 

In addition to the change of inertia, the heavier aircraft needs more powerful flaps (double-slotter vs single slot before) which causes a bigger pitch moment change. 

Therefore a stretched aircraft needs a more effective elevator. 



As for the meaning of "flying together" I assume that this means that the software solution for the 787-10 controls the pitch moment of the aircraft not only by changing the elevator angle, but also uses control surfaces on the wing.