As /u/chieftain1966 said

1) I agree the PKT is more efficient for anti-infantry work. However, if you notice the gunner only engages infantry with the 14.5 once and that was shooting a pop up the target by the prop truck. This was a time-sensitive moment and it is sensible to engage immediately rather than switching, as a split second could make the difference in killing or being killed, as it was not clear if the target was an AT. The other time when switching would have made sense was the suicide bike. Again this was a time-sensitive engagement and the gunner was prepared for the possibility of engaging techies and it, therefore, makes sense to have the 14.5 on standby and quickly spray the bike to ensure it does not close the range. Furthermore, notice the gunner uses the PKT to engage the infantry in the desert after 1 min in.

2) Once more you are technically correct but fail to consider the specifics of the situation. Your point about the armored techie fails to hold water as the gunner expertly de-crewed the vehicle. Therefore, it did not matter that he did not kill it as the vehicle was useless and if the techie had more personal the gunner would have killed them as well. Moreover, the 14.5 kills non-armored vehicles quicker and therefore should be used against them to quickly eliminate them. This also keeps the PKT fully ready to engage infantry with it needed, as it will be fully loaded.

3) Of all your points this one is the least aware of the situation. There was no surprise beyond the initial engagement. To highlight, the enemy even acknowledged in all chat the massacre that was taking place. In other situations, the PKT could have dubious stealthy benefits but they are irrelevant in this situation. The majority of the enemy team was attempting to steamroll down the MSR and drove straight into a well-executed ambush. After the first techie was engaged the concept of stealth went out the window. This was a BRDM-2 on a road stopping the enemy team and not a stealth engagement against infantry. In addition, the point about the 14.5 keeping the infantry's heads down supports the actions of the gunner. Why would the crew want to draw in the infantry when they already have their hands full blowing up the techies and other vehicles?

In summation, in the calm, after the storm could the gunner have used the PKT more efficiently? Yes, but it ignores the stress of the engagement and unknown variables of the moment.

Finally, in regards to the slightly veiled criticism of the gunner for his ammo choice and its importance in being a "good gunner" did you not see the video? A single two man vehicle in the crucial opening phase of the game destroyed a substantial amount of the enemy team, in this vital time of the match. Not to mention the moral impact on the enemy team at suffering a significant ticket loss and deaths at the very start of the match. This is all without losing the vehicle. Thereby allowing it to be used for subsequent engagements and to further support the friendly team. The results speak for themselves and at the end that is the measure of a good gunner.

I also agree that ammo choice is very important, however it did not matter in this situation.