Where are Aurcade score rules located?

I think you misunderstood that statement. The description just says that the run follows both Twin Galaxies and Aurcade ruleset for score, only because that user probably submitted their speedrun on both platforms, so the rules explained are a mix of both sites.

Aurcade only lists what you've linked, and I couldn't find any other rule, nor on its forum.

Twin Galaxies, however, wants you to follow these rules:

Setup Screen Menu Options
Sound = On
Difficulty = Normal
Extra Man = 30,000, 70,000, every 50,000 after

Special Rules: Your Record attempt ends when your lives run out OR you defeat the final boss for the first time as it's possible to die after delivering the final blow, allowing you to defeat him again and again. This is due to the fact he's 100,000 points, which is worth multiple extra lives. Continues are NOT allowed!

Aurcade doesn't mention this, but I feel this is a common lineguide for Splatterhouse submissions, also because it's a blatant exploit.