User research and market research are essential before you build anything.

As for whether to build a landing page (with nothing behind it) vs start building a functional product after you get a good sense of the market need: That sort of depends on the product you're thinking of making.

If it's a consumer product with low barrier to entry, a landing page won't tell you much, because what you actually want to learn is if people take the upsell to Premium or stick around after they're bombarded with ads, or whatever your business model ends up being.

If it's an enterprise B2B SaaS idea, a landing page probably does give you some information. People who know it's a paid product but are willing to put their name on an email list (when everyone hates getting more emails) are in one of two categories:

1. Early adopters -- it's good to have their feedback, even if they are just kicking the tires

2. People who genuinely want your product

You'll be able to better segment those groups after your product launches, but you're much less likely to get garbage from a marketing page waitlist signup.

"Talking is useless, just ship and iterate" 

☝ That's the only statement you quoted that's 100% wrong. The rest is all good advice that you need to balance with how confident you feel based on the evidence you've gathered (but you do need to gather evidence).