[…] almost any slope Defeats her modest hopes. (6-7)
As a synonym of "humble," "modest" has positive connotations—unpresumptuous, reasonable, discreet. But here the word is linked with "defeats," triggering other, less positive, connotations of "modest"—meek, unsure, unassertive. Come on, Turtle, time to play "Queen of the Mountain." It's a competitive game, and you don't have to be so polite!
[…] never imagining some lottery Will change her load of pottery to wings. (12-13)
In Aesop's tale, the tortoise never expected to grow wings and fly across the finish line. But as she plodded along, don't you think she still had the goal in mind, still had hope that winning was not totally out of the realm of possibility? The fact that Ryan's turtle is too humble to imagine a dramatic reversal in her situation does not necessarily mean that she is too meek to contemplate more measured success. Or does it?
Her only levity is patience, (14)
One synonym for "patience" is "humility," so "patience" is not an unexpected word to find near the end of the poem. But we keep bumping up against the odd pairing of "patience" with "levity," which means "cheerful humor or lightheartedness." There's no way around the implication that humility (patience) has its own mysterious joys and rewards.
The sport of truly chastened things. (15)
Though "chastened" often has negative connotations (punishment, suppression), it can also mean "purify" or "refine," implying growth through suffering. Joined with "truly," which connotes authenticity, "chastened" lends the humble turtle an air of seriousness and dignity. Still, even in the last line, the poem won't let us get all somber and self-important—just as "patience" in the previous line was paired with "levity," "chastened" is paired with "sport."