What you read appears to be an inaccurate mishmash of the consequence for making an excess contribution to a designated Roth account which is not timely corrected with information about correcting excess contributions to a Roth IRA, two different types of Roth accounts.
An excess contribution to a designated Roth account in an employer plan like a 401(k) must be corrected by a return of excess contribution by April 1 of the following year, otherwise the excess contribution and its attributable net income are subject to taxation upon distribution, making for double taxation of the amount of the excess contribution. Distributions from the designated Roth account after the April 1 deadline, no matter ho long after the distribution occurs, consist first of that excess contribution and attributable net earnings until the excess is fully resolved.
Excess contributions to Roth IRAs are treated differently. You have until the due date of your tax return for the year for which you made the contribution, including extensions, to obtain a return of the excess contribution to avoid the 6% excess contribution penalty you would otherwise incur. Distributions after that deadline correct the excess but are only taxable to the extent of the attributable net income required to be distributed; the amount of the excess contribution itself is not taxed again. Another difference between and excess contribution to a Roth IRA and a designated Roth account in an employer plan is that the excess Roth IRA contribution is subject to the 6% excess contribution penalty each year that the excess remains in the account. You also have the option to apply the excess contribution in the Roth IRA as some or all of a subsequent year's Roth IRA contribution. That option is not available in a designated Roth account.
Elective deferrals are employee contributions to the traditional account in an employer plan like a 401(k) and are subject to the same rules and double taxation as with excess contributions to the plan's designated Roth account because they are not excludible from income in the year the contribution was made.
