The way my lawyer explained it to me was the same as yours did - the bankruptcy removes any obligation to every creditor that you owed as of the filing date, and presentation of the discharge paperwork to someone you forgot to list would be sufficient proof that you’re no longer liable for that debt if it ever popped up. (You can also amend a filing to include additional creditors, but I don’t know anything about that process though; what it costs, if it can be done anytime before or after discharge - maybe someone else can chime in here.)
My notice of discharge appeared on my credit reports within a few days. After the discharge, every account should be reporting with a $0 balance and noted that it was IIBK. If you had late payments *prior* to filing BK those can (and often do) stay on your credit report unfortunately, but they can’t report any new late payments that are after your filing date. You’ll want to wait a few weeks after you receive your discharge before going through your credit reports to make sure they’re all reporting correctly (the BK7 to 700 thread has some very helpful info about this!). I watched mine closely before and after filing and they were all over the place as accounts were updating, a couple even disappeared entirely, but were all pretty much reporting correctly a few weeks after discharge.