From what I can find, the liquid was a ‘sensitizer’. PETN is hard to detonate, in and of itself.
Other passengers reported hearing what sounded like firecrackers going off. They may have been just that, firecrackers.
If so, they likely would have worked, had he mixed the sensitizer into the PETN completely.
Where he got it? Could have been from the ‘well-dressed man’ who got him around customs and onto the plane.
Or it could have been the other passenger who was removed from the plane.
We will probably never know.