Music from Trinity is not copyright property of present day musicians.
They do not say it is: they say that their performance is.
I used to work for an art publisher. We might make a reproduction of a painting two hundred years old. The painting was not in copyright, but each and every one of our printed reproductions was our copyright. Anyone could reproduce that painting, if they took the trouble to obtain access to it and get the necessary quality photographs. If they tried to short-cut the process by basing their reproduction on one of our prints, they were in trouble. There is more to copyright than meets the eye.
Music may be divine: artists have to live. It goes without saying that the greatest musical performances are "divine" (according to how each one of us might understand it) but divinity, at least that which is manifested through human endeavour, has many practical down-to-earth aspects and, in the context of this subject, I think it is a red herring.
<cross-posted with mahakavi, who puts it better>
In fact, it may be tweaked more than a bit: In a performance of any given kriti, especially if it is a major piece in the concert, the alapana is the composition of the performers, as is the kalpana swaras, and the neraval (variations on a theme) is shared. I'd go so far as to say that the most exquisite hights of a great concert are likely to be in the Manodharma (did I spell thatright? It's past even my bedtime) aspects. The composers, trinity or others may be the inspiration or the launching point, the compositions are a vital base of the music, but surely we go to see the creativity of a musician when we go to a concert?