Plutonium-238’s half-life is 87.7 years, Americium-241 is 432.6 years. Which… is almost 5 times longer, so… not sure why that’s cringe?

What's cringe is the word "staggering". Natural radioactive isotopes have half-lives on the order of billions of years. All elements heavier than iron are created in supernovae. Billions of years have passed since the novae that created that heavy elements now on earth. Anything with shorter half-lives is no longer around. (More correctly, one should talk decay chains.)

What's staggering is that these isotopes are available at all. They are artificially created in nuclear reactors. Mass production of Pl-238 began only during WW2 for bombs. That's almost a half-life ago. The shorter half-life makes the availability of Pl-238 much more impressive.

I believe they’re referring to the fact that it’s not an element of major topic. This is the first time I’ve ever heard of it.

There are over 100 named elements. I don't think I could name half of them. Americium is relatively prominent because of it's use in smoke detectors. And while I'm at it: Americium is the element. Americium-241 is a specific isotope; a specific variant, chemically identical to other variants but with slightly different physical properties.

There are a number of isotopes suitable for RTGs. It's a matter of trade-offs. There's half-life, which is basically how fast the properties of the material change. There's also energy density and how bad the radiation is for the device. And always, there's cost. Fun fact, in Chernobyl they did try robots, but the electronics could not withstand the radiation. People don't withstand it either, but there's a lot of them.

"Small" meaning "author of article seldom thought of it before."