Yes, they did use transparent shadows. You can tell because the images are not GIF's, they're actually a sequence of PNG images overlayed atop other PNG images. There's the base satellite image of the area, then the frames-of-explosion-with-transparency overlayed atop them. You can confirm this by right clicking on the image and choosing "open image in new tab", which will (usually) open this final frame in a new tab.
I tried to save one of the "gifs" to share with a friend and was quite upset that I found myself with a png. Interesting technique. Do you think this was done to have better quality animated images? GIF compression isn't amazing (relative to WebM at least)
Very gratified they didn’t just post these as rendered gifs.
Imagine the social media scare you could generate in a year or so after everyone has forgotten about this eruption by sharing a graphics.reuters.com URL for one of these animations that appears to show a satellite image sequence of a massive mushroom cloud obliterating the Korean Peninsula or the Sinai with a breathless ‘LOOK WHAT REUTERS JUST REPORTED!!!’ caption.
It’s really important for reputable sources posting images to think about the fake news potential of them getting shared out of context.
Probably more along the line of they have some library that uses PNG sequences. Maybe even as simple as a slideshow with short durations. It's a newsy site. They don't have a lot of control over what their CMS/publishing platform can do. Someone probably had a clever idea on how to use something existing in a way not envisioned when created.
[0] - https://graphics.reuters.com/TONGA-VOLCANO/lgpdwjyqbvo/cdn/i...