Then perhaps you need to consider who you are talking to instead of where you are talking.
You have then already failed if I have to read a guide to even start the game. There should be in game help, pop ups or other directions to point a new player in the right direction.
This person is clearly a fan of the P&P and knows how it works but it appears there expectation of what a video game should be like has been formulated by... video games!
In this case probably single player and theme-park video games which are pretty self explanatory and only need a brief tutorial to get you going. Pretty easy when you've already played 100 other games almost just like them.
Sandboxes are catered to players who like depth and detail. That means a steeper learning curve. They also have much fewer games like them and may be the first game of this type the player has tried, which also makes the learning curve a bit steeper.
You're argument to them essentially boils down to. "You like the P&P and this is like the P&P SO YOU SHOULD LIKE IT MORE." An approach more along the lines of "This is probably different than most games you've tried and a steeper learning is the price we pay for a more detailed and interesting game, it's industry standard for comparable games at this stage of development" would have a higher chance of working.
Well... once this game is detailed and interesting anyway. lol