There is no magic. The confusion is because one tends to look at only the DSB/SC of the L-R at 38 KHz.
The Disc output has L+R, and the DSB of L-R all together.
The L+R adds up with the modulated L-R resulting in a 38 KHz AM with 2L and 2R as the top and bottom envelopes.
One can see it very clearly in an oscilloscope that the Disc output is a 38 KHz AM that does not have a symmetric envelope.
Another point to be noted is (though it is not part of this debate), even if the FM is weak, Stereo reception is possible since the 19 KHz pilot plays no role.