I found an Airbus presentation which says this note was added because there was a case where a crew had the flaps locked. They had CONF 1 and applied TOGA to go around but didn't get SRS mode.
So Airbus now say you need CONF 1 and gear down to engage SRS.
Why the QRH says "may not engage SRS" if you don't have the gear down is still a little confusing. Why the "may"; surely with the gear down it does engage and without it then it won't engage.