Melanoma in adults is typically tied back to acute blistering sunburn events that happened before the age of 18 (as you seem to be aware). The most common sites of melanoma are chest and back in men and legs in women, areas that are more often than not covered by clothing and not in need of sunscreen.
Cumulative sun exposure on the other hand causes the other types of skin cancer - not melanoma, which this vaccine is targeting.
The most common chemical sunscreen ingredients cause cancer themselves and are wildly overdue for FDA review and removal (that has been held up for political reasons). For those who wish to wear sunscreen I recommend non-nano zinc mineral sunscreen such as Thinksport. Look for "non-nano" and zinc oxide (not titanium oxide, which is often nano in part and also a carcinogen) on the label.