I may be missing something fundamental here, but I'm not sure what that post has to do with Russia or propaganda. You say it's obvious they didn't die last week

I may be missing something fundamental here, but I'm not sure what that post has to do with Russia or propaganda. You say it's obvious they didn't die last week, but it wasn't obvious to me upon reading - perhaps because I'm not American. But if there's no date in the text, maybe your mum just didn't remember the original event, thought some soldiers had died, and she would be honouring them with some copypasta? I guess I don't see a need for a conspiracy, Russian or otherwise, to get people re-sharing posts on Facebook that explicitly say "if you don't reshare this you're a bad person".