Your understanding of より is perfectly fine. But you have misunderstood 今まででいちばん多かった前の年. This just says 2019 is the worst year in the previous statistics, and the past tense is used simply because the statistics was made in the past. However it does not imply this record has been already broken. That is, as of today, 2019 is still the worst year, and 2020 is the second worst year.

_{(English past tense ("was X") tends to imply it is no longer X now, but Japanese past tense tends not to have this implication.)}

Your understanding of より is fine; it just means "than ～" here. But you have misunderstood 今まででいちばん多かった前の年. This says 2019 is the worst year in the previous statistics, and the past tense is used simply because the statistics was made in the past. However it does not imply this record has been already broken. That is, as of today, 2019 is still the worst year, and 2020 is the second worst year.

_{(English past tense ("was X") tends to imply it is no longer X now, but Japanese past tense tends not to have this implication.)}