One death from overdose is more impactful than one death from Covid, but if the increase in number of deaths from Covid (0 -> 400,000/year [1]) is much greater than the increase in deaths from overdoses (78,000 -> 100,000/year [2]), then Covid is the primary cause of the decline. Deaths from accidents also only increased 39,000 -> 43,000/year [3].
Let's say the average age of someone who dies from Covid is 75. Their life expectancy is another 11 years. [4] And someone who dies from an overdose or accident at 30 would have had another 48 years on average. There would have to be (400,000*11)/48 = 92,000 (rough math) additional overdose and accident deaths to come close to the impact Covid has had on life expectancy decline.