My take is that a hand pump does not produce moisture as fast as an automated pump. Far more important to have a moisture trap when using an automated pump as they are going to pull moisture out faster because they are generally compressing faster. I've hand pumped exclusively since I got into PCP airguns (a good 10 years now) without any evidence of moisture in any of the tubes. I haven't bothered using any inline filters. Stopping every 40-60 strokes, bleeding the line and letting everything sit a few minutes goes a long way to keeping the internals dry. On the plus side, most people are gonna want to take a break around then anyway.
Edited to add that once you have moisture in the tube, you're going to get that kind of corrosion where two different metals are making contact, for instance brass and aluminum.
 
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I'm betting he's just not pumping fast enough to actually produce precipitated moisture in a quantity sufficient to get out of the pump and into the rifle. Most hand pumps have a reservoir that collects moisture as it precipitates from the compressed air. If you aren't filling that cup during the pumping session, it should blow out when you vent the pump to remove the hose. There's probably still a bit of humidity in the compressed air inside the tube, but it would only be a problem if it condensed.
 
There are so many variables in moisture problems.
1) Location, do you live in an extremely humid loacation? That will affect moisture getting into your guns.
2) How you fill. This covers a lot of ground. Is the gun above the low point of the fill hose? That can keep moisture from "rising" to the gun. Do you have a cotton filter to stop liquid from moving into the gun? Do you use desicant? Do you fill fast or slow, take breaks, etc?
3) What do you fill with? A hand pump is hard to pump fast enough to build up substantial heat and precipitate moisture in a large volume. Do you use a slow compressor Ie., a GX or similar? They don't fill fast and build a lot of heat. Does you compressor have any moisture mitigation built in?

That's just a few of the problems that can crop up. The truth is, and combination of them can cause problems, but all can be mitigated should the need arise. I filled with a GX for a long time, and never had moisture in any of my guns. I live in WV where moisture is not a big deal, so that's likely the saving grace. I now use N2 fed to a GX and that works like magic and no, absolutely NO moisture, so no worry.

If it's me, I would choose a method of pumping up my gun ie. hand, compressor, scba, whatever, and then use it and check my gun for moisture from time to time and let that be my guide on filters, and moisture mitigation. You can't argue with results.