It’s a Wordpress based site using the Central theme.
Pretty much everything can be done with a framework based theme such as Foundation or Foundry. The sub menu could be a simple tab stack of which there are many examples. You would never get it exactly the same but it would be pretty damn close and most users wouldn’t be able to tell the difference.
If you really like it, it would be cheaper to buy the theme and build it in Wordpress to be honest.
One of the things the cited webpage does NOT do well is handle the tabs when you scaled down to iPhone size. It works, but it’s a bit awkward/ugly.
OTH the Switcher stack recommended by @SteveB can do a very elegant “switch” from tab to accordion format when scaled down. The result is much more pleasant to look at.