That's the difficulty of establishing REE potential within the...

That's the difficulty of establishing REE potential within the clays, let alone whether they're also easily extractable ionic-bound REE clays.
Presumably the REE content of those samples outstanding won't be anything like the +1,000 TREO that the initial reported samples assayed at, but being a bulk mining exercise the actual values will very much impact on total pre-resource tonnage available. Thus if the analyses come back quite low it won't be a complete failure for project economics, but it could impact on the size of an operation down the track.
The positive aspect of TAR's REE project is that it holds a huge tract of 100% held ground that appears to be highly prospective for REE's, and the values are commonly elevated from or close to the surface.
Once these lower XRF results are calibrated to total REE it should (maybe!) provide a better outcome to future hand held XRF determination. But for resource estimation all sampling will require a more reliable REE determination.
Even a clear correlation between absolute REE content and kaolinitic clay content may not be sufficient, as it is so important to be able to establish what, if any, correlation there is between recoverable REE and the analytical technique developed.
I imagine there's a fair amount of learning on such a project as this for any company.