FYI, in my experience if you have much below 12v system voltage, an E53 does not start...no clicking, nothing. I have found this over the last month, keeping a 30w solar panel on it with a charge controller that will kick in well into single digit voltage. I do not get enough sun to keep topped at 12v, and if it is not, nothing happens. Once you get down to below ~8v, your door locks won't work. That includes rear hatch you would need to open to get a new battery in there.

The test was probably a load test which is the best way test a battery and it tested good even with its age. Of course there is the possibility the test was done incorrectly. And, since you had it out it was a good time to get a new one when it is that old.

However, I wouldn't assume all is well because you haven't mentioned any symptoms, such as slow cranking, prior to the stone dead morning. Since it is so easy I would at least unlock the cluster and monitor alternator output and check battery condition each morning for a week or so.