At the beginning as it is used in reference to Drop Pod Assault is definitely in context prioritized over at the start of turn references to reserve rolls.
Drop Pod Assault itself references first selecting the Drop Pods to arrive by drop pod assault then the remainder are placed into reserves as normal, what happens then when you have an army wide or formation rule that permits all units to potentially arrive from reserves on turn one including all the other Drop pods do we then ignore the sequenced order spelled out in the rule for Drop Pod assault and read things in the opposite order first rolling for units that arrive from reserve on turn one and then selecting the remaining half of the drop pods that failed on their reserve rolls rounded up to arrive via Drop Pod assault because start of turn and in the beginning mean the same thing? No of course not because common sense and context matter. In this case they are clearly telling you Drop Pod assault is carried out prior to anything else dealing with the reserve phase because the units arrival occurs before reserves as normal even happens.
Spearhead alters reserves as normal in the sense that may re-roll and arrive on turn 1, but that wouldn't mean they magically ignore all that is spelled out in Drop Pod Assault in the sense that it is occurring prior to reserves as normal. It is gerrymandering of rules for gain but hey you find an opponent that is fine with it good luck.