I have noticed this pattern but have always been a little skeptical because there seem to be obvious signalling reasons to make this claim irrespective of its truth.

But there are also equally obvious signaling reasons to make the opposite claim -- i.e., I Am Doing This Work That Is Really Hard Because It Is (And Therefore I Am) Important And Prestigious.

And some people do make that claim. They just usually don't have much to show for their efforts, by comparison to the people making the other claim.

They may enjoy some parts of it but much of it is still effortful and not the most enjoyable thing they could think of doing at any given moment.

The sensation of "effort" is the sensation of your mind trying to escape whatever you're actually experiencing in the present moment, whether it's because you dislike what's happening or you wish it were something else.

In the absence of that escape attempt, there is no "effort" felt, vs. what you might simply call "exertion" instead. Things just are, and doing happens.

Could you clarify exactly what you think productive people mean when they say they 'love their work' and explain what leads you to believe that it is literally true?

I think perhaps you are reading "love" as something like "receive pleasure by", whereas the intended meaning is more like "create pleasure through".

When I do something nice for my wife, I "love" what I do in the sense that I am doing it with love -- investing myself in it for the sake of the result. This is pleasurable, but not because the activity itself is necessarily pleasurable. It is what I bring to the activity that makes the difference.

To put it another way, "love" in this case is an active verb, where one is the do-er of love-ing. Not a passive verb, in the sense that we might say, "I love this weather we're having", but more like the love in "I love you".