Let's try and settle this shall we?
In the past, the trophy was presented when the second and below placed teams were mathematically incapable of catching the team placed first. Therefore, they were crowned the champions. The rules have changed this season so that the winners lift the trophy on the final day.
We are the champions of this season. We won it. Saying that we didn't is like saying Chelsea or Manchester United could catch up with us and finish first. Sorry, it won't happen. Why? It's mathematically impossible, therefore, we are the champions.
Oh, and the Oxford Dictionary classifies 'Immortal' as:
constatnt, long-lasting.
Now, if long-lasting isn't ONE WAY of saying unbeatable, then we're not immortal. But I do believe it is.