The attribution of "bushwhacker" as coming from the Dutch "forest keeper" surprised me, as the compound word bushwhack already makes sense as a description of the activity and a quick search indicates both bush and whack were already present in English prior.

The Oxford English Dictionary etymology of the word just indicates it is a compound formed from those two English words[0]. Online Etymology Dictionary does mention a possible connection to the Dutch however[1]. It would be interesting to know why etymologists make the connection to the Dutch.

[0]: https://www.oed.com/dictionary/bushwhacker_n

[1]: https://www.etymonline.com/word/bushwhacker

Edit: it seems the English sense of the word bush as meaning "the wilderness" came to it from the Dutch bosch via the American colonies[2], maybe that's the connection.

[2]: https://www.etymonline.com/word/bush#etymonline_v_18128