The article claims "One of the most surprising of these techniques involves the Roth IRA, which limits most people to contributing just $6,000 each year."

That's sort of correct (the number is $8,600) wrt direct contributions but direct contributions are not the only way to fund a Roth IRA.

Roth IRAs can also be funded by converting a regular IRA or by rolling a Roth 401(k) into a Roth IRA. A regular IRA can be funded by rolling a 401(k).

This is relevant because the contribution limits for 401(k)s, roth or otherwise, are significantly higher than the IRA contribution limits.

The actual yearly contribution limit for both Roth 401(k)s and regular 401(k)s is $70k. Employers are allowed to cap the employee contribution at $23k, but the employer+employee contribution limit is $70k and employers that don't contribute their max ($70k-$23k) can let employees make up the difference. (And, if you're over 55, the $70k is actually $82k.)

Google "backdoor ira" to find out more.