Baloney!
The deaths due to combat in the Napoleonic Wars (1803 to 1815 and one of the bloodiest of the period) were 2,500,000 to 3,500,000, from all causes. About 291,000 per year.
The deaths due to combat in World war I were 8,400,000 (1914 to 1918) from all causes. About 2 million per year.
The deaths due to combat in World war II were 22,000,000 (1939 to 1945) from all causes. About 3.6 million per year.
Combat deaths in WW1 and WW2 were an order of magnitude higher than in the musket era.
Artillery has always been the queen of the battlefield. In fact in the musket age, if you were going to engage an enemy further away than 150 yards, you brought up the artillery. So, the percentages of the casualties due to artillery is probably the same +/- 10% or 15%.
No, musket are not the most deadly by a long shot.
Since, I can't be sure that disease figures aren't accidentally included, I will append with this: Medical science and general care about soldier's health and hygiene were much better in 1914, and further improved in the 1940s, we can safety say that the non-combat death due to disease were smaller.