Do 5% of startups actually reach 1B valuation in 4 years? No. Uber took 10 years to IPO. I don't know the average, but I would imagine based on a cursory search that it's closer to 7 or 8 years for highly valued companies in the past decade. And 5% seems extremely high for billion dollar IPOs.
Also, only 2 startups are compared in the NPV calculation. What about joining a BigCo / FAANG? That would really put it into context.
On the other hand, the math gets slightly better if you assume you leave after some amount of vesting. I thought danluu had a post about this, but can't find it. Staying in a startup until the bitter end is almost never the optimal choice, especially if you have inside knowledge of its progress.
https://tldroptions.io/ - This was posted a few years back, and while it doesn't give tell you the likelihood of a particular size of exit, it does help give an idea of the type of dilution and final equity value (with no discounting though).