I probably would not have put it quite as harshly as Patrick but, this is the main forum, not the beginners section and there is some credence in what Patrick is saying. There's nothing wrong with starter strips and feeding a swarm but ... it was a small swarm and a bit of TLC is required. A dummied down full hive is fine but the spare space needs to be filled with insulation and the space isolated from the bees with close filling dividers (you can call them what you like as far as I'm concerned but there is a difference between a dummy board and a follower). Whether they were overfed, underfed, lacking a queen that was fully working, had too much space, too little space, were diseased, infested or were just one of those colonies that were never destined to thrive we may never know.
Neglect is too strong a word but the beekeeper does have some responsibiltity to check the progress of new colonies (regardless of where they come from). You cannot just put bees in a box, leave them to it and hope for the best. The OP is a 2 year beekeeper - anyone who says they know everything after two years is kidding themselves - we all make mistakes and this one is a learning curve .... there's no obvious answer to the OP's original question - the bees are dead, move on, learn from it and look to the future.
There is nothing more depressing for a beekeeper than to see a colony that has died out; it's not something any of us like but it happens - we will blame ourselves and that's possibly enough without our fellow beekeepers adding to the remorse.