Brass of a given cartridge from different manufacturers can vary widely in terms of the thickness of the webbing, case wall, neck wall, etc. It can also differ in terms of its metallurgical properties; i.e. composition, hardness, etc. Finally, it can differ in terms of the consistency to which it is formed/machined. Each of these properties can contribute what we might think of as the overall "quality" of the brass. Lapua is considered by many to represent the pinnacle of brass technology with good reason.
Although I would agree with the referenced statement that your Federal brass is probably not the equal of Lapua, that does not necessarily mean you can't get the Fed brass shooting well. In some instances, people refer to the "longevity" of a certain type of brass as synonymous to quality. For others, the measure of quality might be "precision". The specific parameter someone uses to gauge the relative quality of a given brand of brass makes a difference. It may well be possible to develop a load with Fed brass that groups as well as the Lapua, but I doubt it will last nearly as many firings. That doesn't mean you should pitch the Fed brass if you already have a load with it that shoots with acceptable precision. Just keep an eye out in case the primer pockets open up, which is often the first and most obvious sign the brass is used up/finished. I know of no reliable method to predict in advance how many firings can be obtained from a Lot# of brass, there are too many variables involved. If you only get 4X firings from it, so be it. But if there are a few more firings left in it and it still shoots well, why throw it out? You have the Lapua brass and it is not going anywhere, it will still be there when the Fed brass is finished. At the pressure(s) I would expect from your load with 39.0 gr of Accurate 2495, it could be there are quite a few firings remaining in the Fed brass. The good news is that regardless of which route you decide, i.e continuing to use the Fed brass or tossing it, you are GTG because you have the Lapua brass in hand. Just be aware that an optimized load in Lapua brass may end up quite different than one worked up using Fed brass.
With respect to trimming, measure your cases and trim accordingly. I trim .308 cases to 2.005". If you wish to let them grow a little longer for your specific chamber, choose your desired case length, measure the cases at the appropriate point in your case prep routine, and trim to that length when they need it.