Not necessarily via a button but all of the pages were submitted to them by 3rd parties. So, they don't seem to crawl to discover new URIs eagerly, see https://hackertimes.com/item?id=29643506
For example, I host my own ArchiveBox at home (you get fulltext search as a bonus) and it is configured to submit every URL I save to IA: https://imgur.com/a/Yhnxo1W IA considers that to be manual submission not subject to robots.txt rules.
I don’t think that is correct. A lot of it was added via automated methods?