It's common for HTTP servers to accept requests without a host header. It's not usually needed by the server unless you're hardening it (I don't class it as a security issue but some security audits will flag it up if you don't force the server to reject invalid host headers) or running named virtual hosts (which is more common than it used to be thanks to SNI but you still often see a 1:1 relationship between (virtual) hosts and IPs). So Amazon could easily have tested their client on 3rd party servers and still not spotted the problem.

As an anecdote, about 15/20ish years ago I wrote my own webbrowser. Obviously something highly rudimentary albeit browsers were much easier to implement back then anyway. I was too lazy to read the HTTP spec (it was a hobby project and I was young and impatient) so a lot of what I did was trial and error. I too wasn't sending a host header but it took long while before I ran into any sites that rejected my HTTP requests. The web landscape was very different back then though and IPs were plentiful but it just goes to show how servers have coded around bad clients for years.