Thing is that he is assuming that a message would have been put in the
last 50 million years or so. By that time there would be an awful lot
of work needed as animals will have diversified to such an extent it
will be hit and miss whether the right species gets the altered virus
or not.

There is a problem in transporting DNA vast distances because of the
danger of damage through interstellar radiation. Even at the speed of
light the time it takes for the DNA to get here from the other side of
the galaxy/universe there would be no DNA left.

It would be far more feasible to put a message in before the species
diversifies using something to make the DNA from native molecules say
around the pre Cambrian.

It would be cheaper than sending energy intensive messages for millions
of years hoping that people would tune in, or could be used in
conjunction with a lower powered transmission once we know where to
look.