They're probably not pushing even hundreds of Mbps - it's a 2MB site in total, and almost all of it is static, and will be cached by various proxies/etc.
They might PEAK at a Gbps for the first few hours, and then fall off to a long tail of basically nothing.
It's just not big - no serious images, no video, etc.
And it's US only, not global. And most people will only hit the site one time, order and it's done.
And even so, $2500/mo is 'free': just the paperwork to put the money on contract to the provider probably costs more than that by an order of magnitude.
A general rule of thumb is CloudFront is $2500/mo for every 100 Mbps of traffic you have, not counting origin or per request fees.
I would expect they are pushing multiple Gbps with every household in the US trying to order tests.