It wasn't that simple: in practice the arrival of the 286 in 1984 was very much only the beginning of the PC's escape from the 640KiB/1MiB limit. It wasn't until some time after the promulgation of the DOS Protected Mode Interface in 1989 that it became routine for PC applications to have unrestricted, straightforward access to all of RAM. (I'm very much relying on resources like https://news.ycombinator.com/item?id=17274672 for the details here: I was never a serious DOS user or a DOS developer.) Still it would definitely be overstating things to say that "PC memory maxed out at 640 kilobytes" in 1993 or subsequently. It does seem possible that the author did mean to say "640 kilobytes", but really just as a snarky jab at the PC platform: PC software really was still relying on DOS extenders to make use of "extended memory" in 1993, even if they were by then doing a solid job at it.

Another possibility: apparently https://www.os2museum.com/wp/windows-nt-3-1-and-os2-memory-d... all x86 machines released before maybe 1993 are restricted to 64MiB, even under Windows NT and other fully 32-bit OSes, because the BIOS simply can't report having any more RAM than that. By early 1993 the first "service" allowing the reporting of more than 64MiB of system RAM (up to the full 4GiB supported by x86) was starting to appear on some BIOSes.

Apparently the effective memory limit for Windows 3.1 is 256MiB https://web.archive.org/web/20150518111050/https://support.m... while for Windows 95 it's approximately 480MiB https://devblogs.microsoft.com/oldnewthing/20030814-00/?p=42... . So it's plausible that Rosenthal had one of those in mind, even if 640MiB is not exactly the right number (and even if you'd actually hit BIOS or wallet limitations first).