Is it really jpeg artifacts? Seemed more like the artist just uses allot of non-anti-aliased pixels and strokes, it lacks the usual noise, blocking, or color banding present with higher jpeg compression (Though I do see some blocking on flowers, Yuuka seems fine)

Same here, i dont see any Artifacts on Yuuka or the letters. Just the flowers in the back. So its the usual "Clean forground" and "blurry backround" kinda thing. Anime does this alot too. Usually to emphasis on certain characters.