Having translated the reviews to English with google translate, there are enough comments about the owners being friendly and discreet ( and their first names being referred to) which would probably/possibly have made me wonder and ask some questions.
But having said that, I do think there is definitely some less than honest manipulation of photos and what they describe online.
For starters, the main photograph, of the pool and the building behind it, would suggest that the building was the house for rent. But as you have said, it actually isn't. Careful perusal of ALL the photos might have alerted you to the purpose of the building behind the pool - but not sure that it would have done at the time. Different now you know though. There is only ONE photo, showing the actual accommodation exterior and it is from the air and well buried in the numerous other photos.
Secondly, the property is not named, so you can't google reviews of it anywhere other than the ones shown on the booking page. You need a property name to be able to do that. As for the other reviews on the site, 15 in the last few years is hardly going to be representative of anything. I would be entirely suspicious that they were all 10/10. They may be genuine, but possibly cherry picked?
I think all you can do though, is speak to them about the vehicle noise and hope they are reasonable. It's a shame when the expectations of your holiday let have not been met, due to somewhat suspect advertising