I have this clementine cutting that I placed in to coco coir and when I tug on it it doesn’t come out. And now it’s flowering !
Is this an advisable method of propagation?
Well it's not a usual way to grow a new clementine plant, they are usually grafted onto a different rootstock. Grafting of plants, apart from the need to recreate the exact same fruits, is done for other reasons too; the rootstock may supply vigour to the grafted plant, or may restrict its overall size over time, or cuttings of the plant itself may prove very difficult to root.
If your cutting has produced roots, it would be interesting to pot it up and see how it does over time - the production of flowers is not a guarantee the cutting has roots though, it may just be that the cutting you took was already programmed for flowering, but certainly, pot it up and see what happens, especially since it seems impossible to extract the cutting,which might well indicate it has rooted into the coir.
Information on grafting here,should you be interested https://homeguides.sfgate.com/graft-clementines-83489.html