Diesel engines---all engines---can be rafted and isolated into cells. You can do further isolation at the gearboxes and shafts. As the engines are likely inspired from MTU designs, you're looking at a V16 here. This is not a straight four diesel here. You know how increasing the number of cylinders in an engine then spacing the timing and the weighting of the crankshafts can smooth out an engine, why a six cylinder is smoother than a four cylinder, an eight cylinder smoother than a six, a V12 even more smoother than a V8, and then you arrive to a V16. Then these engines are isolated, and rafted, the engine mounts are hydraulic and so on. These features were incorporated into both the 054A and 056 series. Not saying that all these make the power train smoother than a gas turbine, which completely lacks reciprocating movement. Just saying that the diesel powertrain can be made smooth enough that the PLAN deems it sufficient for the mission.
If 054B has a gas turbine and IEP, its not going to be called a 054B. So far the PLAN Type no. is based on power train and platform, and "054B" sounds to me like it will still share the power train and hull platform from the 054 albeit with heavy changes on the upper hull structure, the same way the 052D, 052C, 052B and 052 all share the same power train structure from the 052, with big changes happening on the superstructure, sensors and armament of the ship.
Reasons for using diesel engines comes down to thermal efficiency. A marine gas turbine will reach about 36 to 40%. A marine diesel engine will reach as high as 48 to over 50%. There are marine engines used in the commercial field that boasts thermal efficiency as high as over 60%.
