I think Bill is correct.

I tested on two Macs, both with Safari version 18.6, using the https://chipguide.themogh.org URL. The newer macOS Sequoia 15.7 works fine. The older macOS Ventura 13.7.8 shows the "Connection Is Not Private" message as Mr. Deems sees. After poking around and comparing the two machines and doing a little online research, I tried the following and it worked for me (your milage may vary, and only try this if you are comfortable poking around on your Mac):

I went to this certificate issuing authority (who is updating certs and is likely why older machines are having issues) and downloaded a cross-signed certificate. *Note that when you go their website you'll get the same "Connection not private" message and have to "Show Details" and click the "visit this website" link.

https://support.sectigo.com/IS_KnowledgeDetailPage?Id=kA0Uj0000004IrB

Scroll down to the section titled "What is cross signing?" and look for the intermediate certificate ending 4893.

Click that link and it will download the certificate into your "Downloads" folder. Open the Downloads folder, locate the file (should be named 11405654893.crt. Double click it and it will install the new certificate. Now trying loading the chip guide again. I hope this works for you as well.