True, but according to the article, "Lyme disease accounts for about 80 percent of the tick-borne illnesses in the U.S."
Plenty of people who get Lyme don't get the rash, and don't necessarily even know they were bitten by a tick. Deer ticks are tiny and you have to check yourself thoroughly to see them. An effective vaccine would be very helpful.
They probably mean the known tickborne illnesses. When illnesses are unknown it's often because they're rare, so it's probably a good estimate.
In any case, Lyme disease is very common and on that basis alone, a vaccine would prevent a lot of suffering, regardless of what other suffering remains in place.
