You should check this out with your host. The County has been careful not to imply that they want to disrupt existing bookings though they haven't ruled it out.
To be legal, they must show a NUC number or a permit number prominantly on the advert. If they don't show that, they are not legal.
Problems could arise. If the host gets fined between now and your stay, it is almost certain that they will suspend all future rentals as the fines for subsequent infringements are much higher.
Some hosts may be willing to circumvent the law in this way to offer shorter stays. There is still a risk that the activites are brought to the attention of the inspectors and that the host will have to cancel your booking.
Advertising is a new violation under this law, but actually staying in a home for less than 30 days remains a violation. The DPP will continue to monitor for occupancy violations. Also, Governor David Ige recently signed House Bill No. 807 (Act 114), which makes it a misdemeanor offense to knowingly make a false statement to a county inspector.
We thought it was too! However, this is not the place for a political or philosophical discussion. There are various places for such discussion including the FB group "Oahu short term rentals 30-day rule discussion group" and OSTRA: http://www.oahushorttermrentalalliance.org/
These regulations are made by the County which is only Oahu. However, Maui has had its own regulations for a long time requiring a minimum 180 days for a rental with exceptions for properties with a permit. Big Island requires registration of short term rentals under Bill 108 ( Ordinance 2018-114). For legal rentals on these islands, you will see a Permit or Registration number.
This seems extremely unlikely. However, the County's own FAQ states "actually staying in a home for less than 30 days is still a violation" which implies that the guest is violating the law. However, nowhere else in the law itself or in the County's publications is there any indication that the guest can be deemed to be at fault, so our interpretation is that the statement above is simply careless use of language.