Consider yourself one lucky puppy. If the chain was truly too short, and you somehow forced the shift, you would at the very munimum bent the rear axle. Or, you might have destroyed the derailleur and/or the hanger, and possibly a chainring, and even a chainstay.

Advice to anyone reading -- do not work on any derailleur system before confirming that the chain is long enough to loop the two largest sprockets, with enough slack that the RD still has room to give. Or if the RD is off, loop the big/big pair with an inch to spare.