What implications does younger volcanism have on the Moon's thermal evolution?
A younger lunar thermal history implies that the Moon was still undergoing a contraction and cooling process, which was initiated when it began accreting material together. Evidence has shown that the moon has no tectonic activity, so all geologic features are volcanic.

The mare are a more recent example of this activity, and have been shown to have fairly young ages, as you indicated. The moon still has some internal heat from its formation that may be leaking out through convection and cooling. 

Even though the moon has a relatively small mass, it didn't lose its internal heat as quickly as the earth because there was no tectonic activity to facilitate it fast enough, and convection is a slow process.

As the moon cools, it contracts, creating sharp ridges and cracks in the surface (see features known as 'scarps', or 'garden'). These contractions contribute to the "deposit" of material formed in the images that paper discussed. As the cooling surface shrinks, it breaks the crust through a thrust fault, which pushes some material higher and shoves other material lower.



What may appear to be river channels on the moon are also lava flows. These were initially underground lava channels whose roofs collapsed to expose these subterranean features. More recent incidents of molten material rising to the surface and cooling can contribute to some of the younger ages recorded in the basaltic rocks.



Tldr; Evidence of younger volcanic activity confirms existing theories for the contraction and cooling process of the moon, just that it didn't end as soon as we initially thought