This is an awesome project! There have been others who have made their own computers using as far as a 486 (https://news.ycombinator.com/item?id=26407783 ) but making it IBM PC compatible really takes it to the next level. The only thing that would make it more amazing is if he didn't use existing chipsets and implemented all the glue logic on an FPGA (like the aforementioned 486 board) or discrete TTL.
The Pentium was when Intel started to get too secretive, I think; there's relatively little of the low-level information that's needed to create hardware like this compared to previous CPUs. I believe modern PC designs are all entirely based on a reference design from Intel so if you could get access to one, it would be possible to make your own motherboard, but signal integrity and such are going to make the PCB layout much more difficult. That said, there are plenty of laptop schematics out there... and (although a group effort) some Chinese managed to make a custom motherboard replacement for a Thinkpad: https://news.ycombinator.com/item?id=15274644
The Pentium was when Intel started to get too secretive, I think; there's relatively little of the low-level information that's needed to create hardware like this compared to previous CPUs. I believe modern PC designs are all entirely based on a reference design from Intel so if you could get access to one, it would be possible to make your own motherboard, but signal integrity and such are going to make the PCB layout much more difficult. That said, there are plenty of laptop schematics out there... and (although a group effort) some Chinese managed to make a custom motherboard replacement for a Thinkpad: https://news.ycombinator.com/item?id=15274644