There is a point device bug found a while back ago. With this your savefile gets overwritten by the demo.

For a step-by-step guide refer to below:
1. Launch LoLK, version a (this does not work in version 😎
2. Select your category in PD (in this case Marisa Normal since it is one of the three replays shown in the demo)
3. Go to the title screen and wait for the demo
4. (optional) Fastforward the demo to save time
5. Select your category again and you'll be in the chapter the demo ended at

I would like to know if the usage of this bug is allowed or not.


No, the use of this bug would not be allowed for use in speedruns. By resuming from a save point, that would not count as "starting from the beginning." The demo run would be considered different from the initial run that you started. Thank you for your question.