It’s certainly possible to win - and it’s not a problem of strategy. There are any number of valid strategies. The problem is in execution. The reason the premades could win is at least in part because they could listen and competently execute a strategy. When they lost, it was usually in part because of a failure to do that.
With a pug group, it’s not impossible to do it… it’s just exceedingly hard, because you essentially need to win the lottery and get players that are of a minimum competence.
It’s further difficult because you don’t have as much room for error and setbacks as the Horde does. If you fail to take a forward graveyard before SHGY falls, you’re SOL. If you lose your forward graveyard and can’t take it back immediately, you’re SOL. Even all the way to the end when you’re trying to pull warmasters, if someone screws up and pulls 3-4 at once, you can get wiped combined with the Horde mass-recalling - and woops, looks like nobody was watching relief hut which just got capped, and you’re back at your cave. Etc, etc…
It further does not help that the Horde attitude these days is essentially to try and deny the Alliance any honor whatsoever, to the point of ceasing offensive operations just to come hunt you all down if you’re somehow managing to make any progress at all.