Their success is not directly from SEO but SEO amplified their word-of-mouth. They have also acquired brands that has helped but never ever forget - a killer solution to a killing pain point is what sets you up for success!
Yes, HTML-only bookmark manager like BookmarkOS is very good especially if you used a disparate amount of platforms like Windows desktop PC, Linux laptop, and iOS Mobile.
I would recommend Bookmark OS. It's not open sources but offers full text search, full page screenshots, and other neat features https://bookmarkos.com
I've seen many services like this come and go. As great as some of them are, having to trust a 3rd party with a full text archive of your complete web history is not a good value proposition. Not only due to privacy but also longevity, I don't want my archive to depends on this company remaining honest while also becoming successful enough to be around 10-20 years.