> These two passages are in contradiction to each other.

By default, a thread signal can receive any signal and the signal handler implemented in C will be called in the context of the thread.

CPython ensures that a signal handler implemented in Python and installed by signal.signal() will be called in the main thread. It uses a signal handler implemented in C which only store the notification and will be the Python signal handler later (as early as possible).

pthread_sigmask() can be used to mask some signals (or all signals) on a specific thread.