The issue lies with the fact that search warrants are usually issued with a very narrow scope of investigation, in this case the warrant would have been to locate the F100 ute (or parts thereof) buried on the premises. Once that has been located, that's it, no more searching or excavating allowed. They can't go 'well we think this person is sus, let's just dig up the whole backyard and see what we find'. There needs to be specific intelligence before a warrant is issued.
Obviously if evidence of further offences had been uncovered while looking for the ute then they could broaden their search but police obviously have very specific information that none of us are privy to and the ute was all that they were after, at this stage anyway.
The way that the media releases have been worded, and of course I'm just speculating here, it appears to me that locating the ute may have been more an exercise in testing the credibility of the evidence given at the inquiry. I can't remember the exact wording but the statement was something along the lines of 'we think we know why the vehicle was buried there, we just want to confirm our suspicions'. I'm not sure that the evidence uncovered is the smoking gun (although of course evidence is evidence), but it may well lend credibility to the version of events heard at the inquiry. If there was any shred of evidence that a body may be located on the property, I guarantee that they wouldn't have stopped at looking for the ute. They would have got a warrant to search for a body, personal belongings or anything else connected to the case.
Reading between the lines I think they know exactly what happened to Novy, but I highly doubt she was buried at the Mt Nathan property.