Through my employer I have access to pre-tax 403(b) and 457(b) plans and in addition an after-tax "defined contribution" (DC) plan, see https://www.myucretirement.com/resources/articles/0055. I was told by a Fidelity representative that I can do "mega backdoor Roth conversion" (MBRC) of the DC plan contributions to a Roth IRA as follows: the principal goes to my Roth IRA and any gains can be moved to the 403(b) account. So far so good.
However, my understanding is that if I didn't do this MBRC, and let my money sit and grow in the DC plan bucket, then any gains will be taxed as regular income (upon withdrawal after retirement). Is that true? If so, my confusion is: Isn't that "double taxation" in a way? For instance, if instead I had put that after-tax money in a brokerage (instead of the DC plan), then upon withdrawal, the gains would be taxed as long term capital tax gains, whose tax rate is typical much lower.