وَتَأْخِيرُ الْعِشَاءِ إِلَى مَا قَبْلَ ثُلُثِ اللَّيْلِ
Translation: And it is recommended to delay Isha until before the first third of the night has elapsed.
Here the first third of the night is meant, not the beginning of the last third. Isha’s valid time continues beyond this preferred interval until true dawn.