I believe they tend to OSRS gold work on things first if it's very likely to pass a poll or whenever they can use the resources elsewhere. In this case it was directly according to a popular suggestion from 6 weeks ago so they could suppose it would be well received. But people complain that they don't have enough details and vote it down. So they need to perform a substantial quantity of the work before bringing it into the survey.
I can not confirm whether or not it's accurate, but Jagex continues to be on record dozens of times stating they do not put substantial dev time to pre-poll content. They mostly follow the suggestions and roadmap and receive community input pre-poll. I am pretty sure group ironman was announced at runefest 2017 and has been supposed to be released 6 months down the street at the most recent. Idk why Jagex makes many empty promises every Runefest; it is like they make all these programs and if they get into the dev function they realize it is actually going to become a whole lot more work than they expected but they are fearful of being transparent with the community about it.
The over-promise and under-deliver because it gets people to buy the premier club each year, and as soon as they have their money out of that it's like they no longer give a fuck about the actual updates. Jagex already has the skilling contract frame laid out a long while ago. For example, PoF contracts have been inserted on the fly. It was just a ninja update that required a couple of days. In case Jagex needs, they can add contracts to all the other skills on the fly in both RS3 and OSRS too. The question is do they have better implementations than skilling contracts.
OSRS has multiple brand new coaching hotspots
PoF contracts came together with 120 farming. And honestly, there is not a huge reason to perform them if you don't need trim. They're only great for reputation, and reputation is really only worth finishing for completions sake. Meanwhile, OSRS was stuck with the same old same old way of construction training for 7 years before now.
Saying aquarium is not the same old way of instruction is very questionable. Additionally, by your logic of"brand new hotspot = new method of training" OSRS has multiple new coaching hotspots such as gnome benches for EHP training. Is not Mahogony Homes using the same old method of coaching for OSRS too? It absolutely is. At least Aquarium brings out more methods for adding fishing to it. If you're taking a look at brand new methods of construction coaching, wait a couple of months before RS3 includes fresh brand Construction methods in the fashion of buy RuneScape gold Safecracking in RS3. This has just been revealed in the livestream continuing now.