If these two decide that they wish to support his bike endeavours, in gratitude for what he's already done, for any other reason, or for even no reason at all, then there's nothing wrong or "unethical" with him taking these donations. They're all voluntary, his mod is in a playable state for the previous version last I checked, and he DID say that he's going on the trip round the various countries, and that updates will halt to pretty much nill. He ain't pointing a gun to anyone's head, is he?
I have to disagree with you. He made no such post that I am aware of on his Patreon. His last post was on December 9th, 2016 when he released version 1.23. It states that 'next week' he will be doing another bugfix and balance update and adding new content. Nowhere in his patreon posts is it indicated that he's leaving.
He's well within his rights to pursue his biking, obviously. His patreon though is for (quoting here) "tl;dr: Masterwork DF. Weekly updates. Every Friday. Support it. Thanks.". That is on the front page. It does not mention "also supporting my biking hobby". If he wanted to open a patreon for people to donate to his biking, that would be fine too but if he no longer plans to support the masterwork patreon for the time being (as it has been nearly a year now with no word) then he should pause it until he picks it up again. You can easily suspend patreons and then resume them.
Instead he's chosen to continue receiving money donations for something he's not doing, for a year. If any other charity or donation-fed organization did that, they'd be burned at the stake. It's unethical.