Office really chugged on the PCs of the time though. We can debate whether modern Excel actually delivers enough more value than historical Excel to justify being as more resource-hungry, thus slower to load, as it is. But historical Excel appears fast on modern hardware, even in emulation, because the CPU, RAM, and permanent storage have had 30 years to evolve since it was released. Contemporary 386s and 486s would not have been that snappy.
If this were a commercial project then I could understand the complaint.. but this is just a small, for-fun project and they have little motivation to put the extra effort into support for all browsers.
Bellard (yes, him) already had a working VM of Windows 2000 in the browser around a decade ago, with no specific "support for all browsers" (whatever that means):
I started from Windows 98 and always loved the icons. They actually represented the application and purpose. These days they are more focused on looking modern. Lots of times they are not even distinguishable between each other.