They are talking about a timespan of 4 months. At my office, most people still work from home, but of the 10-15 people that show up regularly (and we have plenty of space to social distance at the office, with CO2 meters usually around 450-550 ppm), half of them caught Covid during the June-July wave here. All of them caught it somewhere else, not at work, usually from their kids or family.
So, we still got the e-mails "hey, I was at the office Monday, started feeling sick Tuesday and test came back positive", but since I was nowhere near those people and I still try to avoid common areas too much, who cares.
As long as they don't force their employees to cramp inside a small meeting room with 20 people, chances are much higher they got it elsewhere than from work. Unless they can really identify hubs of infections in teams.
Because if there's something people are really good at, it's minimizing their own risk of contamination outside of work. If you have kids and a family, I think it's a x10 risk vs getting it at work.