"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.