how come decoding is more work intensive vs transcoding ?

With JPEG you can scale down in power-of-two factors by just not processing the higher-frequency components.

You're still left with the non-lossy compression step, so one could do something like "unzip", repackage data throwing away the high-frequency stuff, "zip".

Wouldn't surprise me if you can do something similar with h264/h265.

I'd be surprised if that worked for interframe compression like video but even if it still did, and extremely well, encoding to the resulting format still has to fit in and that's where the majority of the work should be even when you just decoded the source normally.

I think they mean transcoding as an additional step upon already decoded frames.