Yes, I meant to say that the default is withholding is 10%. Thanks. I've made the correction. Because an RMD is ineligible for rollover you don't have the mandatory 20% withholding that would normally apply to distributions from qualified retirement plans, so you can use Form W4-P to decline withholding or have some other amount withheld.
The default withholding that Fidelity mentioned is for periodic distributions (distributions in the form of annuity), not for nonperiodic distributions from retirement accounts. Since you mentioned that this is an RMD from a 401(k), I assume that it is a nonperiodic distribution (even if you have a standing order for Fidelity to make the distribution on a particular date each year). See 26 U.S. Code § 3405:
https://www.law.cornell.edu/uscode/text/26/3405
