Really surprised to see so many open positions at once.. Checked from google that Jagex has little over 300 employees, so normally offering this many jobs at once could only mean two things:
1) their business is growing super fast (unlikely), or
2) they have problems keeping their staff and people are going out faster than new people are coming in. Usually that's not the best sign.
However, there is also a possibility that they aren't actually looking to fill all these positions asap but they are just sort of "fishing" new talents even before there is an urgent need. That might be useful because the gaming industry is still growing fast and potential employees have so many other companies that could also use their skills if Jagex doesn't catch them first.