I wouldn't blame the rain. My silkies are out in it every time it rains and they've never had anything wrong with their feet like that. Many people's chickens like the rain.
I'd soak it in warm Epsom salt water for 15 minutes or so. Then look at it again. Feel it if it's hot or just normal. Is it hard or soft? I didn't read the article
@Wyorp Rock posted yet, as it might mention PRID or drawing salve, but that sometimes works well for bumblefoot.
I'd just do the soak first and determine what that is for sure.