If he is in trouble, why would he announce it like this? Perhaps he's not that shrewd. And it makes no sense for him to bluff if he is safe. In a wave, WI-6 is most certainly on the table and Wisconsin is one state where waves reach high.
I don't see this seat being on the table even in a wave, and keep in mind that I think netting 50 seats or possibly even more is something that's within the realm of possibility for Democrats under the right conditions. This seat is R+8, has been held by Republicans for 50 years straight, and is held by a new incumbent who's unlikely to retire and let the seat be open. The district is a mix of working class-whites and college-educated voters who are more movement conservative types rather than traditional moderate Republicans that are the weakest link of the GOP under the rise of Trump, so they still wouldn't be that inclined to vote Democrat even if they're dissatisfied with Trump.
Seats skewed this favorably in one party's direction rarely fall in waves when incumbents are running, and the demographics of the district are friendly to Trump
and tea-party types. Maybe if this seat had been open in 2006 Democrats would have had a chance, but not anymore. The only situation I could envision this seat being competitive now would be if the seat was open in a wave election and the Democratic candidate had extraordinary appeal with the working class voters. If a seat like this falls in a wave with an incumbent running, Republicans will be losing in a landslide of closer to 100 seats rather than just 50 seats.
I'll gladly eat crow if Grothman actually gets ousted, but I wouldn't hold my breath on it.
