Probably, but it could have been a mitigated problem. Its not entirely unusual for the NARSUM to state something that is out of place from the rest of your medical record. The PEB reviews everything, so they may have not taken great weight from that incorrect statement.
Example: My NARSUM stated my sleep apnea was a significant problem and it explained my somnolence issues. The PEB cited my sleep study and the timeframe of the symptoms appear as conflicting issues and declined to say the sleep apnea was unfit, inviting me to either explain the timeline better or associate the somnolence symptoms with another conditions. So I can say that at least in my case, the PEB doesn't just parrot the NARSUM.
Is it to late to do something about it?
No, the BCMR can correct errors. I don't think asking them to simply correct the NARSUM makes sense, but if you can show how that and/or other incorrect information led to a decision that effected you in a bad way, they may well be able/willing to fix it. So really what you want to establish is your PEB results were wrong, and to do that you show them how the MEB results the PEB used was wrong.
This further feeds into Mike's questions. The NARSUM having this incorrect information is likely to not have any real effect. If applying for CRSC, the main point of combat related, they won't see the NARSUM unless you send it in.
The problem with TBI being 10% and grand mal seizures being 100% is due to pyramiding rules, which are universal and unrelated to a combat related determination.