But how you imagine that would work? How will a longer timespan help?

Let's picture that we literally took 10 000 top human engineers and scientists, with all our human knowledge, into dolphin bodies, on another planet with no human artefacts. So, our dolphin people now need to somehow develop a way of writing down their knowledge underwater, which they can only do very laboriously because they haven't got hands. They can write very large letters with immense energy expenditure per letter. They can barely store any knowledge. They also got short lifespan and sharks to worry about.

And on the tools side, you need tools that are good enough so you can use them to make better tools. That generally requires ability to harden things - make something while it's soft, let it harden, use something softer to crack apart something harder. And to get that started, you need hands, because without hands you can only make the kind of tools that doesn't help you make better tools.

If you can't make an improvement in any single generation, you can't make any improvement in a thousand generations either.

Meanwhile, a planet populated with those same scientists and engineers in human bodies - hell, dog bodies, cat bodies, elephant bodies - would've had it all sorted out in no time. They'd have steel, electricity, running water, radio, and so on, in less than a generation - hell even 10 people can do that.

(assuming they all cooperate).

The gap due to the body shape and environment appears utterly immense. The only hope would be that dophins would evolve much greater than human intelligence and come up with something that we can't come up with (e.g. mind controlling some animal with hands).

edit: That is not to say a small number of top scientists and engineers would single handedly create industrial manufacturing, but that is to say they would re-create pre-industrial village level technology and then hand-make many important bits of 20th century technology. You can take a 16th century blacksmith's forge and make an electric generator in there, a spark gap transmitter, a coherer receiver, a carbon arc lamp, and the like, using most basic materials and hand manufacturing techniques. Indeed that's how the early instances of all those things were made - by a small number of top engineers, often in their spare time, without advance knowledge.