This is somewhat disingenuous because it neglects to mention that the 90 day exercise window is typically the result of a company offering employees incentive stock options (ISOs). A 90 day exercise window is required by the rules associated with ISOs. Startups can't change the law.
When a company allows exercise after 90 days, the options become non-qualified stock options (NQSOs), and those are subject to different tax treatment.
In reality, the favorable tax treatment of ISOs often doesn't benefit startup employees, so there's an argument to be made that startups should just offer NQSOs anyway, but that's neither here nor there.
The big lie is that extending the exercise window has a high probability of being meaningful. Yes, it's true that companies are taking longer to deliver liquidity, but a lot of companies simply aren't going to deliver liquidity, ever. For those that do, in today's market, where valuations skyrocket early and late-stage investors trade valuation for significant downside protections, many employees will find that their equity isn't as valuable as they expected.
If a startup really wanted to stand out and reward employees differently, it would look at alternative approaches, such as bonuses and profit sharing plans, including profit sharing plans that contribute to a 401k. I think a lot of people who are not new to the game would be attracted by alternative structures and incidentally, these would probably do a lot more for retention.