Yeah the study seems to make 4 assumptions:

1.) The life's only metabolic source of energy is glycine

2.) The life is analogous to glycine metabolizing organisms on Earth

3.) The only source of glycine getting deep into the ocean is through these rare impact events

4.) The life can only survive in the deep subsurface ocean

Speaking of ocean, water is mentioned several times in discussing Titan but the hydrologic cycle analogue consists of various hydrocarbons.

But to the fourth point, I wonder to what extent radiation is a factor in mutation of biological material. It turns out Titan has a rather low surface irradiation coefficient due to being rather distant from Saturn's radiation belt, a weak induced magnetosphere (rare for moons!), and the ions are mostly water-based coming from Enceladus while Jupiter's radiation belts are largely sulphur ions coming from Io.