You're definitely asking the right question, and IMO the answer is "sortof." It's not that impressive in the grand scheme of DDoS, given major attacks will sustain 100 gigabit traffic[1] sometimes for days on end. But 20gbit/s would likely be enough to take down a small & unprepared business.
20 Gbps definitely isn't very impressive given the current landscape, but it can probably take down many medium-sized businesses as well, if they have no DDoS mitigations. Even a large one, if they somehow had no mitigations or no decent security team.
20Gbps is actually still impressive if it is a set of valid requests like the parent seems to imply when doing the calculations. 20Gbps via a reflection technique is easy, but also relatively easy to filter and just requires a large pipe. 20Gbps of traffic doing legitimate requests that you can't immediately and trivially filter/rule out is still an impressive feat right now.
It's high, but DDoSs over 500 Gbps have been observed in the past 2 years. So relatively speaking, it's not that extraordinary.