Your number is on a list of real numbers with real identities associated with them that was sold to them. Data brokers sell this information daily. They already know your number is real, but in order to comply with the law, they have to provide you with a legitimate option to opt out, so you will actually stop receiving correspondence from them if you ask them to stop (it is legally required). If not, they could be subject to a fine, but you’d obviously have to file a complaint with the relevant regulatory body for that.

If you don’t reply to opt out, they will continue sending messages to you in the future. It costs them almost nothing to do, so even if they didn’t know your number was real, they would do it anyway. Most of the people who donate from these messages don’t reply through text message anyway.

Pretty sure they won’t keep messaging me after I blocked their number and reported spam

True, but if you get a new phone and your blocked numbers list is reset, or they send messages from a different number, then you could get them again in the future. I see this often because there are multiple people in that campaign that will all reach out to people with their own phone numbers. Opting out prevents that for legitimate donor campaigns (you are removed from the list for all of the solicitors associated with that campaign), but obviously not for scams. There is no harm in doing both, and I would recommend that (it’s what I do).