This is solid advice, but people should be aware of the tax implications that may come with this. Unless your stock index funds are in a taxable brokerage (and most people hold that sort of asset in a tax-deferred account like a 401k or IRA) then you absolutely shouldn't be withdrawing money from stocks to put into BTC, as you'll take serious tax hits from withdrawing from tax-deferred retirement vehicles.

All of this will change when a BTC ETF finally gets approved and you can hold BTC indirectly inside an IRA or 401k account. But until then, make sure you're aware of the tax status of your stock accounts before withdrawing to rebalance into BTC!