I believe OSRS gold they tend to work on stuff early if it's very likely to pass a poll or whenever they may use the resources elsewhere. In this case it was directly based on a popular suggestion from 6 months ago so that they could suppose it would be well received. So they have to do a significant amount of the work before bringing it to the poll.
I can not confirm whether it's true, but Jagex has been on record dozens of times saying they do not place substantial dev time to pre-poll content. They largely flesh out the suggestions and roadmap and receive community input pre-poll. I'm pretty sure group ironman was declared at runefest 2017 and was supposed to be published 6 months down the road at the latest. Idk why Jagex makes many empty promises every Runefest; it is like they make all these plans and if they get into the dev function they realize it's going to become a whole lot more work than they expected but they're afraid of being transparent with the neighborhood about it.
The over-promise and under-deliver because it gets people to buy the premier club every year, and as soon as they have their cash from that it is like they no longer give a fuck about the real updates. Jagex has the skilling contract frame laid out quite a little while ago. For example, PoF contracts have been added on the fly. It was only a ninja update that took a couple of days. The question is do they have better implementations than skilling contracts.
OSRS has multiple new training hotspots
PoF contracts came together with 120 farming. And honestly, there is not a massive reason to perform them if you don't want trimming. They are only good for reputation, and standing is actually only worth finishing for completions sake. Meanwhile, OSRS was stuck with the exact same old same old way of construction training for 7 years before now.
Saying aquarium is not the same old way of training is very questionable. Also, by your logic of"new hotspot = new method of training" OSRS has multiple new training hotspots including gnome benches for EHP training. Isn't Mahogony Homes using the exact same old way of training for OSRS too? It was. At least Aquarium brings out more methods for incorporating fishing into it. If you are taking a look at brand new ways of construction coaching, wait a couple of months before RS3 includes new brand Construction methods in the fashion of Safecracking in buy RS gold. It has only been revealed from the livestream ongoing currently.