What CLI command can I use to obtain MAC CHIP model?

Can you elaborate on what you are referring to as the MAC chip model? You can refer to this article for more details.

If you mean NIC, then according to 400F datasheet it is driven by NP7.

https://www.fortinet.com/content/dam/fortinet/assets/data-sheets/pdf/fortigate-400f-series.pdf

You can find the same with this command.

diagnose hardware deviceinfo nic port1

Apart from this info, I don't think you can find somewhere official if the FG NICs are Intel, Broadcom or something else. However between us I think it is Intel.