The sovereign-to-sovereign IHT exemption is not just to do with zero IHT at the point of death. Its fundamental purpose is to avoid the erosion of the monarch's wealth and keep it intact (that's literally not the case for anyone else).
The exemption also applies to the private wealth of spouses of monarchs. Hence it applied to the Queen Mother, although she was never a monarch herself.
That would be equivalent of Joe Bloggs setting up the suggested trust and then not dying for 7 years; and then when his mother-in-law dies and her estate also benefits from the existence of Jo Blogg's trust and 7 years' survival.
Even if they (the m-i-l) had failed to make the necessary IHT provisions (eg, the suggested trust and then not dying for 7 years) for themselves.
Additionally, if someone puts all of their assets into a trust (as per the suggestion), there will be legal and/or tax consequences and effects (which depend on the trust) either in their lifetime and/or for their estate and/or their eventual beneficiaries.
For a start, the inherited assets are now in a trust (because that was the suggested solution); furthermore there may be tax consequences further down the line, such as exit charges to take them out of the trust or restrictions in the ability to claim other tax reliefs. At the very least, there is an an added level of complexity in the ownership of the inherited assets.
This is quite a different effect than under the exemption.
Under the exemption - the estate remains intact with the same legal ownership structure as before and with no other consequences or conditions attaching to it (that weren't already there before for some other reason) in the future.
Incidentally, the exemption is also a lot simpler to comply with. The "rules" are covered in under 300 words in the MoU), and is pretty much a blanket exemption without any other conditions (other than being a monarch or consort, of course).
On the other hand, Inheritance Tax legislation extends for roughly 250 paragraphs plus schedules and the estate needs to be compliant in all of the relevant conditions to be effective in avoiding IHT. (And if the person makes an error in their estate planning, the beneficiaries can do little about it)
The exemption is not subject to legislation - it is voluntary and only subject to a memorandum of understanding. So if an unforeseen situation (or perhaps a mistake) were to arise (or even, if they just changed their minds), it can be amended or completely set aside without a change in law.