I don't get it

Mitrios said:

I don't get it

It's just Lily stating the cold fact

Still mind-breaking Lily over that other kind of spring, that (real) Letty.

P.S.: Seems trolling is the one thing both the real Letty and Osa-Letty have in common.

no no, you don't understand.
see Lily's line : "isn't it cruel to the people who are looking forward to spring?"
in this case, Lily is only referring to the actual season of Spring, but "Spring has come" also means "getting a first love"
and see Tani Takeshi's portrayal of Letty Whiterock. A lonely, depressed spinster who wants to get some romance in but never gets any.

So: Lily is unknowingly using an expression that hurts Letty as well.

That little grin from Letty does suggest, though, that she is playing the "spinster yearning for her spring" card to confound Lily, just like the last time it happened.