USMA 1994 is generally correct. However, to be clear, the fact you are "qualified" (i.e., 3Q) with a nom doesn't necessarily mean USNA Admissions is immediately looking to "slot you." Every year, many 3Q candidates with one or more noms receive a turndown.
The key with MOC noms is "winning" the slate -- USNA typically looks at a competitive MOC slate and decides who "wins" that slate. Assuming that person is 3Q, he/she will almost certainly be offered an appt and slotted to that MOC. However, the process can get more complicated. Let's assume Candidate A has a Pres nom and an MOC nom. Candidate B only has an MOC nom from the same MOC as candidate A. Candidate A may be the best candidate on the MOC's slate. But, if USNA really wants Candidate B, they could slot Candidate A to the President and Candidate B to the MOC. There are all sorts of permutations of these scenarios.
If a really well qualified candidate has a Pres nom, as 1994 says, USNA may offer than candidate an appointment early in the process -- being willing to slot that candidate to the President. However, doing this with too many candidates too early in the process starts to limit USNA's ability to form the class as folks with appointments typically stop the nom process (in this case with MOCs). Thus, as a hypothetical, if they gave 75 Pres nominees appointments before the MOC noms come rolling in, they only have 25 more Pres slots available. If several MOC slates have multiple great candidates, some of whom also have Pres noms, there aren't many Pres noms left for these candidates.
That is why there are folks at USNA specifically tasked with managing the nomination process. For candidates . . . don't worry about it. You can't control it. You can only make yourself the best candidate possible and try to obtain as many noms as possible. The entity to whom you are slotted is absolutely irrelevant to you once you have that appointment in hand!