Ask HN: Do we really need to LEAN test everything?

My friend and I are developing an app (with an aim to help people with business networking and creating meaningful business relationships) It's a simple app and I think we should build an MVP. However, everyone seems to obsessed with the LEAN THINKING and they tell me that I have to figure out a way to test the market with a landing page or something similar.

What do you think? Should I really go for it and test the market with a landing page for an app? If so, do you have any ideas on how to 'put it out there'. Since no one downloads an app from a desktop and we are not on Apple Store or Google Play, this will be a challenge. Thanks!