Here's why his statement doesn't make sense:

First, he advocates putting 20% down in order to avoid paying mortgage insurance. That amounts to putting down a lump sum of $20,000 - $30,000 in order to avoid paying $100 - $150 bill per month. Question: if you save $100 per month, how long will it take you to save up $20,000? Answer: 200 months (which is 16 years and 8 months). And that does not count inflation. If you include inflation over time, it means that at $100 per month in savings, you will never recoup the value of the $20,000 lump sum you dropped in order to save $100 per month.

A better use of that $20,000 would be to invest it in a retirement account. Even a low-ish performing 5% return would result in a hell of a lot more money earned over 16 years than saving $100 per month. (I'm mobile and cannot calculate the specific numbers, but you can research what an initial $20,000 investment would yield after 16 years at a 5% compounding interest rate).

Second, he said that the 20% down will hedge against the risk of the property depreciating. First of all, you only gain or lose money when you sell the property. If you buy a house for $100,000 and it depreciates in value to $80,000, it doesn't actually affect you, unless you decided to sell the house at that time. If you decide to continue owning the house, the market will shift and property values will again change in the future. In 10 years, the house could be worth $110,000. So the decrease in value only affects you if you decide to sell when the value for the house is less than what you paid for it.

Alternatively, if you did decide to sell when the value was less than what you paid, then using your own money to hedge against this loss doesn't make any sense either, and here's an example to illustrate why:

Let's say you bought a house worth $100,000. You put down $20,000 from your savings and borrow the remaining $80,000. The housing market turns south, your property decreases in value from $100,000 to $80,000, and you lose your job, preventing you from making payments on your home. Naturally, the bank will foreclose on your house, sell the property for its current value ($80,000), and use that money to pay off the loan balance.

Now, the bank hasn't lost any money because it sold the house for the amount that it let you borrow; so the bank is even. You, on the other hand, have just lost $20,000. Poof, gone to depreciation. That is an unacceptable loss.

Alternatively, you buy the house, but keep your $20,000. Instead, you borrow that $20,000 from another bank (this is a common practice). Same scenario, market goes south and house is foreclosed upon. This time, the bank that lent you the $20,000 can potentially take the loss for that $20,000, instead of you taking that $20,000 loss. (This part is a little more complicated, but the short of it is that rather than you being guaranteed to take the $20,000 loss if you put that money down upfront, the bank that lent you the $20,000 bears the risk of loss and you may be able to keep that $20,000 safe somewhere else).

So no, putting 20% down on a home purchase is not the financially wisest course of action.