Hi @Changwoo82
This specification is valid only for mentioned conditions, so it can slightly vary at different conditions. The pull up/down device is emulated by a FET transistor on the chip. On previous devices, it was usually specified using pull up/down current flowing though the FET, not by resistance. But you can still expect values around those 20-60k.
Anyway, considering the worst case 60k, it's highly recommended to not rely on internal pull up/down when it is needed. For robustness, it's recommended to use external resistors 4K7 - 10K, especially in noisy environment.
Regards,
Lukas