Combine spaghetti squash, vegetable broth, rosemary, and 1/2 teaspoon salt in a multi-functional pressure cooker (such as Instant Pot(R)). Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 7 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove lid. Remove spaghetti squash and cool until easily handled, 5 to 10 minutes. Scrape insides of squash into spaghetti strands with a fork.
Combine basil leaves, olive oil, pine nuts, nutritional yeast, garlic, 1/2 teaspoon salt, and pepper in the bowl of a food processor; pulse until smooth. Serve on top of spaghetti squash.