On one hand, the basic intent behind this advice very true. Liquidity is worth something; the reason people are willing to forego 3% returns on T-bills, 7% returns in the stock market, 9% returns on rental income, or 1000+% returns on startups is because they either need the money now or there's a non-negligible chance of the money not actually being there later when you need it. (Note also that those asset classes - and their returns - are in inverse order by liquidity.) Anyone who tells you that your stock-options are surely going to triple when the company IPOs or that this ICO is going to make you a millionaire or that owning real estate is a guaranteed way to build wealth is selling you bullshit. And it's very useful to be able to see through that bullshit and appropriately discount it.

But OTOH, people who say that you should value illiquid assets at zero are also spouting bullshit. That's clearly wrong: there are people who get rich off of stock options, or RSUs, or real estate, or cryptocurrency. And you're also strictly better off at a company that gives you $100K + 500 RSUs than one that gives you $100K. I know folks who didn't bother to negotiate for RSUs when they joined Google because they either didn't know what it was or valued it at zero; those folks now have a net worth several times smaller than the folks who negotiated for more stock. This is a poor-person mistake: believing that only those things that you can ascribe a cash value to right now have value.

Like most things, it's worth breaking out of excluded-middle fallacies and understanding that your optimal strategy lies in making a best-effort estimate of some very fuzzy and uncertain quantities. You'll be wrong, but you'll do better than either those who value those quantities at zero or those who believe the estimates of the folks who sell these assets.