But when people colloquially say that wealth people are hoarding cash, they mean what the comment you're replying to is saying and not the keeping cash under the mattress thing. More commonly, people say hoarding wealth.
"Hoarding cash" means exactly what it sounds like - people being overly attached to cash in the safe/under the mattress/in "high interest" savings accounts.
This has been its use for a long time in any publication talking about money.
I know when you say "hoard wealth" you really mean not spend their wealth, because I'm not really sure how you "hoard wealth", as being wealthy basically means you've got stores of valuable assets... or a hoard if you will.
The two phrases are definitely not used interchangeably.
That is not "hoarding cash" - keeping your net worth under the mattress or in a savings account is hoarding cash. Perhaps, at the margin, a money market account.