If you use the appropriate wood, you can wet and sand many times to get a smooth finish. You can burnish it (rub with hard metal) to close the surface well. But it will still stain and absorb smells, just to a lessor extent. You'll notice if you use the same spoon for coffee and tea, or the same spoon for curry and miso soup.

Why would you use a wooden spoon for those? I use wooden utensils for frying and stirring in metal pots and pans. Everything else is just the usual stainless steel type suitable for utensils.

I don't know either, but the article is about wooden coffee cups.

When the wood fibers get wet they swell and become soft. When soft, the surface will be very sensitive to damage. Fibers on the surface will raise and then not return to their original position, causing surface roughness. Repeated cycles of wet/dry will cause cracks.

From experience, even the cheapest wood spoons won't bulge, as long as you don't leave them immerged in water for a long time.

Could that be because they are treated somehow?

They were likely treated with mineral oil, but the finish has been gone for a long time now.

I don't have a problem. if they get a little funky I just sand them down. and let them soak in food-grade mineral oil for a while. same with cutting boards and butcher block tables.

Depending on the climate, it can go moldy very quickly.