First point is that what I say in that quote is incorrect. Sky LLU Broadband and Sky Talk (calls) is not full LLU. It is LLU Broadband plus Sky Line Rental that is Full LLU.
However, to try to answer your question.
When you pay someone other than BT (Retail) for your phone rental
there are two ways they can be providing it. There is no obvious way of knowing which way other than asking/browsing around.
The normal way is using a BT Openreach service called Wholesale Line Rental, (WLR)
. That is what O2 does.
The good thing about WLR is that it counts as a "BT Line" for all ISPs that require one, ie nearly all ISPs. The awkward bit can be that many WLR line providers, again O2 is one, don't allow you to have another ISP's broadband service on the line - it has to be their own. Though apparently O2 do allow you a grace period to move the line if you migrate the broadband first.
The less common, (in number of ISPs but now almost certainly most common in number of subscribers due to TalkTalk and Sky), is Full LLU where the ISP, (technically the Communications Provider - CP), feeds your phone service down their LLU backhaul to some place under their control, and from there into the BT PSTN system, rather than letting it feed into the PSTN network at your exchange which is what happens under WLR.
Because the phone service is much more under the CP's control than with WLR it no longer counts as a BT Line. So migration away means you either have to move to another Full LLU provider, or you first have to get the line back to a WLR supplier, not necessarily BT itself.
Look at it another way which may be simpler. When you have broadband your line is physically connected to the broadband equipment in the exchange.
Under WLR the exchange side of your phone connection is also connected to the broadband equipment so as to get onto the line to you.
Under Full LLU goodness knows where your phone connection gets back into the BT PSTN system. It certainly isn't at your exchange.
To get back from full LLU can mean either paying a reconnection charge of some amount to a WLR supplier for the line itself, and possibly an ADSL connection charge as well, rather than a free migration. Having said that, BT (Retail) at the moment seem to be doing such moves free of charge, so long as you take an 18-month contract for both line and BT Total Broadband.
To many people that seems less than ideal
I'm not sure how clear that is, but the best I can do right now.