Those are actually two different etymologies/meanings. Amazingly, the word “impregnable” itself has two meanings, which are kind of the opposite of each other.
For “impregnate”, it lists the meaning “to fill pores or spaces with a substance” under the same etymology as knocking someone up (which is etymology 2 above): https://en.wiktionary.org/wiki/impregnate
Those are actually two different etymologies/meanings. Amazingly, the word “impregnable” itself has two meanings, which are kind of the opposite of each other.
See etymology 1 and 2 here: https://en.wiktionary.org/wiki/impregnable
For “impregnate”, it lists the meaning “to fill pores or spaces with a substance” under the same etymology as knocking someone up (which is etymology 2 above): https://en.wiktionary.org/wiki/impregnate
That is quite a coincidence