In September, Kemp had an OPS of .856... with an OBP of .298. Do you think a .558 SLG is sustainable? That .558 SLG was in 57 plate appearances; he hit 4 HR and 4 2B. In July, in 59 plate appearances, he hit 3 HR and 2 doubles; his SLG was .426, or .130 lower than September.
In August, in limited use, he was horrible.
That's what really small sample sizes do; one potential home run doesn't clear the fence, and is caught, two hits to the OF are cut off by outfielders instead of hitting a gap, and your SLG is 25% lower.
Also... Not to rain on anybody's parade, but prudence suggests that DD not think of Kemp (or anybody else) in terms of "until Miller is ready." Miller put up solid, but not overwhelming numbers at AA in 2025 - and what he did in 37 plate appearances in AAA is... well, meaningless. Yes, he could open strong in AAA this spring (in the cold of Allentown...), or he could not, as AAA pitchers develop a book on him. We just don't know yet... and assuming a defensive black hole like Kemp is "good enough until Miller is ready" is the kind of strategy I'd expect from the Pirates... not the Phillies.