Portal 2 is just a heavily modded Source engine.
I know -__- and it's not 'modded', Source has been developed over years.
Valve likes to release new engines with new iterations of Half Life.
Even if you want to use that as a generalization, HL and HL2 use different engines, but all parts of HL2 use Source in its form at the time of release. Since they've confirmed the next game will be HL2 episode 3 and not HL3, the engine shouldn't be a problem.
And their production time is heroically long
Indeed. Still, it's been 5 years. That's 1 year less than the gap between HL and HL2