This is good advice but I think it downplays the seriousness of government insolvency and the repercussions with respect to investing.
The worst case scenario for millenials is not just getting 3/4 of our promised social security; it could be far worse.
To wit, the national debt increased from ~$5 trillion to about ~$10 trillion on GWB's watch (doubled). It increased from ~$10 trillion to ~$20 trillion on BHO's watch (doubled). DJT has promised massive new spending on infrastructure, increasing funding for border patrol etc, a massive military buildup, no changes to SS or Medicare, and massive tax cuts - so it is reasonable to believe we are still on track to double the national debt again within the next 8 years. That is something not unlike exponential (and BTW that "national debt" is just the level of outstanding treasury debt; the actual unfunded future liabilities are much much higher).
To cover the massive debts of the government (and maintain the stability of the economy and society) massive amounts of new money will need to be created to monetize government debt. This has already started, as the base money supply in the US has quadrupled since 2009. Velocity is low so we don't see the inflation (except in things like stocks, bonds, fine art, and tech startups) but DJT's Keynesian wet dream should get velocity up at least long enough for inflation to start showing up in the real economy, and even that doesn't get the snowball rolling, it is unreasonable to believe that velocity will stay so low forever and that the new money will not start flowing through the economy.
In any case, in such an environment bonds will get savaged and there will be great economic dislocations. A simple index-fund based asset allocation between US stocks, bonds, and international stocks is not necessarily going to work in the next 30 years as it did in the past 30. Past results are not guarantees of future returns.
That said, I think taking 15% of your income and sticking with a strategy that you are comfortable with is great advice. The best way to get rich quickly is to do it slowly indeed. But investing in the near-to-medium term might get interesting, just sayin ;)