Why does y-20 need to be longer? it needs new engines, not a longer fuselage.
There's also a precedent for having a longer fuselage to increase the volume of the cargo bay (length, specifically), where the extra space can be exploited by having more powerful engines.
The Il-76MF adopted this modification by lengthening the fuselage 6.6 meters. Compared to the baseline MD variant it was powered by the more powerful PS90.
In the case of Y-20, it could be plausible that they are interested in a similar modification.
I'm not sure if the recently flown Y-20B will have a lengthened fuselage, but we know at Zhuhai 2016 they showed a civil cargo variant of Y-20 called Y-20F-100, which was powered by a high bypass engine that many took to interpret as WS-20 as well as a greater length (in the promotional video at 54m long, up to 7 meters longer than the normal Y-20A).
So there's definitely a rationale for why a stretched Y-20 variant powered by WS-20 could make sense. Whether the recently flown Y-20B has those fuselage plugin is a different matter though