I strongly disagree with your conclusion.
DK-30KP2 and WS-18 are not 60s tech. Check out their history.
en.m.wikipedia.org
They are Soviet most advance turbofan engine when Introduced in 70s and a major upgrade in the 80s. The final version of DK-30KP2 with further refined is product of post Soviet era products in the 90s.
Even the CMF-56 which large number of passenger plane flying with are 70s era engine traces back to FW100 engine.WS-20 core derived from this.
The high bypass is even more Insignificant when used on a large transport plane like Y-20 which has huge payload and capacity to install extra internal fuel tank to make up for the low bypass engine fuel consumption. In terms of thrust D-30KP2 and WS-18 are not much far behind from CMF-56.