I don't get how this can be true. $20 000 invested in some company/fund/asset gives you the same % return as $2 000 000 invested in the same way.
Suppose you want your investment portfolio to return 7% a year.
If it has $2,000,000 you have enough cash on cash return to live off. Enough cash on cash return that you might make maintaining your portfolio a full time job.
It's just as much work to get 7% returns on a penny (or $20k) but the cash on cash returns are not enough to make it worth working as a full time job.
Now suppose you $200,000,000. You can buy profitable businesses. You can pay someone else to run them from the cash on cash returns.
With $20,000,000,000 you can pay a team with cash on cash rounding error to figure out how to engineer the system so that small investors usually lose. So that they only win via statistical unlikelihood...aka "dumb luck."
Big money doesn't invest in the same way. Ted Turner had enough money to buy a TV station with the Atlanta Braves thrown in to sweeten the pot. $20k is not that kind of money.
You can buy a profitable business for its future cash flow at market rate. It with adequate capital becomes a matter of not experiencing bad luck versus getting lucky.
At $20k the rate of return for money is swamped by the rate of return for time. There are better uses of time than seeking high return rate investments.
Saving the money means if you stumble upon one, the money is there available quickly. It is also an acknowledgment that you are relying on dumb luck not beating the market.
By which I mean as working capital it might be enough to buy a lawn mower, some shovels, and a trailer and put you in the landscape business. But only dumb luck is going to turn $20k into $50k in five years without a lot of your time.
In real-estate or the stock market and finance in general, $20k is the short stack gone all in. It’s dumb money at the table.
So save it. Stay liquid. And maybe you will see some luck there to grab at some point.