Well you are certainly learning the harsh realities of competitive zones of md'ing locales

If you're in a competitive beach erosion area, the hard-core guys don't wait till low tide. They are there "working the tide out" a mere 2 hrs or so after the high, and following it out. The reason is two-fold:
1) to be the first one there as the erosion results are being exposed following a coinciding of high tides and high swells (which is what produces erosion, and is usually accompanied with storm activity).
2) there are actually some beaches and conditions, where the goodies can get pulled all the way out, or go deep (below detection level) at the low tide time. For this reason, some beach pro's (depending on the "rules" of their particular beach) only hunt in the receding water itself, and will swear that that's where the best goodies are. Where I'm at (central coast in CA), I have not seen that to be necessary. I see just as thick a concentration of targets on the receded wet, that I would have gotten if I'd fought the receding surf in the same zone. But none-the-less, some guys like to hunt in the rolling receding surf, and believe the targets are at their shallowest and most abundant at that time.
As for your other observations:
a) You are right: just because this fellow (who you gathered to be a veteran at it) worked
one direction, and you saw that he didn't work the
other direction, doesn't mean that ...... therefore ...... you can get targets in the
other direction. Because mother nature, after erosion events, groups the targets in specified zones. Pro's will walk right towards those zones, reading the beach landscape. They know exactly the beach features they're looking for: cuts, inverted scallop/bowl shapes, slopes, wet spots that are lower than the rest of the beach, etc.... They don't waste time hitting the high soft spots. And after each beach storm re-arrangement, there will only be so many targets in defined zones. The first guy there can conceivable clean it a pocket, leaving you to find another zone, or wait till another day, etc.... I have seen a parralel or pocket stretch on so big, and so far, that no one person can exhaust it in a day, but most of the time, the mother nature has certain settle zones depending on day.
b) just because the guy was "working fast", doesn't mean he was likely to miss stuff. A real beach pro. moves fast on purpose, to find the concentrations (sloppy zig zags, etc...). He's not looking for individual targets, at first. He's strictly looking for pockets. Only when he starts finding a few, does he slow down to determine if it's part of a pattern. Because if it's just a random one, he's going to resume the pace, and keep moving down the beach. And likewise when he's done, and working his way back to his car, he may appear to be sloppy and moving fast. But perhaps he was just on his way back to his car, swinging as he went. And as for the few light things you found behind him, perhaps it was out of the heavy zone. Ie.: sometimes mother nature groups things by weight. So if he's getting the heavier items (coins, keys, sinkers, etc...) in one zone, and then sees that by moving higher, or lower, that it becomes lighter targets (zinc pennies, foil, tabs, etc...) he's going to make an assesment to concentrate on the heavy zone, and skip the light zone. For example: If I start finding only zinc, foil, tabs, etc.. on the wet beach after a storm erosion, I will be out of there in a heart-beat (or skip that strata/level of the beach, etc..., and seek zones where heavier stuff accumulated). After a really good erosion event, there will be no light stuff at all. It's all washed out to sea in the zone where you're working. Heck, I've even seen times when there isn't even any zinc (because they're the lightest of coins). And since gold is dense and heavy, your odds to increase your gold take, would be to hunt where the heavier targets are getting grouped. Particularly where you start finding lead, because gold and lead are both dense and heavy.