OP might have been thinking of Itanium (ia-64), a different 64 bit PC architecture from Intel. It was shockingly expensive
There was certainly development on ia64 in the kernel in 1999 under Project Trillian [1][2], although the first chip wasn't released commercially until 2001
Alternatively they may have been thinking about PAE, which was released in 1999 and allowed linux kernels to address upto 64GB on a 32 bit processor [2]
PAE was also used for Microsoft Windows 2000 Server Datacenter Edition. Server machines at the time capable of holding 64GB RAM were beasts, IIRC on the order of hundreds of thousands of dollars. A desktop machine with 512MB memory was considered way more than you needed. I used a Toshiba Tecra 9000 laptop with 256GB for mostly Java and C++ development and never felt like I was anywhere near maxing it out.
PAE might not have met Amazon's needs for holding their catalog or whatever. If they were trying to directly address a data structure that was larger than 4GB, they would have needed some sort of trickery as PAE was usually implemented kernel side to provide separate 4GB address spaces to individual processes with more than 4GB total physical backing.
There was certainly development on ia64 in the kernel in 1999 under Project Trillian [1][2], although the first chip wasn't released commercially until 2001
Alternatively they may have been thinking about PAE, which was released in 1999 and allowed linux kernels to address upto 64GB on a 32 bit processor [2]
[1] https://lkml.org/lkml/1999/3/9/48
[2] https://en.wikipedia.org/wiki/Project_Trillian
[2] https://en.wikipedia.org/wiki/Physical_Address_Extension#His...