FURTHER OBSERVATIONS:
First, it is encouraging to see the effort to do real homework. Many others here do the homework as well, but . . . well, no need to go there.
Second, SORRY, this is a tangent, and doubly sorry since I'm not a particularly good writer, I fumble a bit in the articulation, so things tend to go long, as this will . . . at the risk of referencing sources you are already familiar with . . .
As already noted, what you reference and link is not an "operational bulletin." Operational Bulletins are labeled as such and have an "OB ###" For example, see the list of Operational Bulletins for 2017 here:
https://www.canada.ca/en/immigratio...ational-bulletins-manuals/bulletins-2017.html
and there are links there to the archive of Operational Bulletins for other years, going all the way back to 2006. (Which is no where near complete; there was a hugely important Operational Bulletin, OB-407, which dramatically changed how citizenship applications were processed back in 2012, which is not listed; it can be found here:
http://tinyurl.com/95lglo5 .)
There is a separate list of "updates" to Operational Bulletins and Operational Manuals; see
https://www.canada.ca/en/immigratio...ational-bulletins-manuals/latest-updates.html
Both the Operational Bulletins and Operational Manuals are being replaced by Program Delivery Instructions, which the information you reference and link probably is. Hard to know for certain since these are not so clearly labeled as Operational Bulletins are, and previously used Operational Manuals were (some Operational Manuals are still applicable to enforcement related processes, including PoE examinations and enforcement of the Residency Obligation for PRs, for example).
Unfortunately NONE of these resources is an entirely reliable source. Including recently issued updates let alone those from previous years. Which is why I have digressed on this tangent first. And while in many respects they are useful, often very useful, they are not definitively reliable, and definitely NOT official statements of the rules or policy, that is, they are
informational and NOT binding, definitely NOT representative of let alone a statement of the governing law.
What has made it rather difficult to follow is that the PDIs have been adopted and implemented piecemeal, and the organizational presentation is way short of comprehensive, with some exceptions.
Thus, for example, the most comprehensive list of PDIs related to processing citizenship related matters, including grant citizenship and proof of citizenship applications, appears to be here:
https://www.canada.ca/en/immigratio...ew/functional-guidance-on-table-contents.html
Which itself is not an easy web page to find. Unless, like many of the other pages with links to PDIs, it just happens to pop up in this or that search.
Compare this list of citizenship related PDIs to what is provided at:
https://www.canada.ca/en/immigratio...l-bulletins-manuals/canadian-citizenship.html
which is easily reached through links from IRCC's main pages but is a much smaller list of links to grant citizenship related PDIs, and does not contain direct links to some rather important PDIs (such as the one you reference and link), but also does not even link to many pages where in turn one can find links to other important PDIs.
The structure of IRCC's citizenship related PDIs is quintessentially bureaucratic. Odd how that goes, what a bureaucracy does tending to be rather bureaucratic in nature.
Which leads to a discussion about their content. Likewise permeated with bureaucratic generalities, vagueness, and outright inconsistencies. Not quite so baffling and impenetrable as to deserve the "Kafkaesque" label, but other than the more obvious instructions, many are intricate and tricky enough to demand a big dose of caution against reading too much into them, and CONTEXT looms large.
Which in turn leads to interpreting particular information such as what you quote. Here, in reference to the formal background clearances. Leading back to what I offered as a short answer. That, for example, the four-year validity of the "security" clearance probably means that in processing a grant citizenship application IRCC will rely on and not ask for an update to the INITIALLY obtained security clearance UNLESS four years have passed (we do not see this much these days, but back in 2010 to 2014 such lengthy processing timelines were somewhat common), recognizing HOWEVER that other things may nonetheless trigger a referral for an update in a lesser period of time.