Pretty certain they are just mistaken, it's common to have fluid to weep between the divider sections since it's just acrylic and metal block, however the particles that are present is a different story, that might have been something like dust that must have gotten stuck between there which would be my best guess but can't say for sure.
You see this very commonly with pastel fluids where you will will see a small build up of the nanoparticles that get themselves stuck between the two materials.
It wouldn't. The only thing that o-ring on that extender is doing is preventing a leak from occurring between whatever it's connecting. An extender such as this is going to be used to connect two objects of your loop together, be it two fittings, two components, or a fitting to a component for some reason.
Your tubing needs some type of barb or compression point that can create a seal around it. If you're using barbs, you'll also secure the tubing down with a clamp (zip ties work as well). if you're using a compression fitting, the collar that you screw down will create the compression point.You tubing is not threaded into anything in any of the conventional liquid cooling methods I know of.
The threads themselves in any part of your loop aren't going to create a seal of any kind, it's the o-ring between two objects that creates the seal.
A use case example of one of the extenders you're talking about would look like this (note, I don't know why you would ever need to do something like this example, I'm just hoping this gives you a better understanding):
three components ((from left-to-right)
a 90° fitting adapter (one male, one female threading)
a rotary extender same concept at the extender, but built slightly different (two male threadings)
A 45° fitting adapter. (one male one female threading)
when broken down into their own parts, look like this:
the extender in the middle creates a seal between the two flat surfaces on the faces of the neighboring components with the red o-ring. The threads serve to hold pressure on the o-ring, they do not create a seal.
To attach a tube to a fitting (in this case, soft tubing using a barb, as I do not have any hardline tubing fittings or tube to provide an example) you're going to use a fitting that has a barb built into it, (or a threaded adapter (such as the 45° fitting adapter above) with a barb attached to it, which would look like this) or a fitting that already has a barb built into it
The tube fits around this barb, which is ever so slightly larger than the inner diameter of the tube, and creates a pressure point at the lip
The tube is then secured down with a clamp (or zip tie) which prevents water pressure from eventually allowing it to walk off or be blown off.
compression fittings work much the same way, they have a barb inside of them, but the clamp is replaced with a collar that threads down around the tube creating that pressure point and resisting the tube walking off. this tube-fitted-barb would then very likely be threaded into a component (let's say a radiator) where the threads once again pull the fitting against the radiator, and the o-ring between them creates the seal.
The structure of a soft tube compression fitting looks like this:
