Using their numbers, I am not sure how CDC arrives at their estimate of 300K excess deaths, either. The average 2017-2019 appears to be 2833507. With three weeks to go in 2020, according to the CDC there are 2703232 deaths so far in 2020. So using the average of 60k deaths of the previous weeks, we should be arriving at around 2900K deaths, maybe 2950k. That's 120k more than the average, not 300k excess deaths. Jumps of up to 90K more deaths in the yearly account also seem to be not uncommon (among other things because of rising population size).
Maybe there is a delay with death certificates registering, but then the precise numbers don't make sense to begin with, and we'll really have to wait a couple of months for final results.
https://www.cdc.gov/nchs/nvss/vsrr/COVID19/index.htm
12% big enough to worry about - worry maybe, but will it register as a significant event with hindsight (like 10 years down the road)? And the argument of it being "with lockdowns in place" is not really convincing, as other countries with fewer lockdowns have not necessarily fared worse (afaik Sweden does not have more excess deaths than in some especially flu heavy previous years, for example). Also, the claim was that we will wonder about our stupidity of not having more draconian lockdowns.