That article cites the FT as a source without giving a link for reference, I'd like to see the original piece
The original article:
Wealthy borrowers use home loans to bet on stock market:
One broker said a mortgage-free homeowner with a house valued at £10m had taken out a fixed-rate loan of just under £2m to buy bitcoin
This doesn't sound like it's common practice, just an extreme example of what one person did. In the Netherlands, people have been using mortgages to invest in the stock market for decades. Bitcoin goes up faster than stock, so it's the next logical step. Given a large enough number of people, it becomes very likely that some of them do a certain thing.
This is just one of the risk of very cheap debt. How nice would it be to turn that £2m into another £10m? If you would have done this just a year ago, you'd be looking at even more than £10m now.
As long as it's the wealthy people who do this, and they can afford to lose their investment, I see no down side. It's good for Bitcoin's growth too.
The risk is indeed if people start investing money they can't afford to lose. I sometimes feel like: "I wish I would have invested anything I have in Bitcoin at some point in past", which is a very risky thought if you extrapolate past price increases into the future. I can't afford to lose my savings in a stock (or Bitcoin) crash, which is the reason I'm slowly losing it to inflation on a savings account.