No, the filler cannot fuse split ends back together — the only real fix for an already split end is to trim it off. But the filler works on the damage that makes hair look fragile and thin in the first place.
Damaged hair has a lifted cuticle and an empty cortex — the inner structure is hollowed out after bleaching, colouring, perming or heat styling. The filler penetrates through the damaged cuticle into the cortex and physically fills those empty zones with hydrolysed proteins, conditioning agents and moisture-binding substances. As a result, the hair feels denser, smoother and more elastic from the first applications. The thin protective film it leaves also reduces friction during combing and blow-drying, which means less mechanical breakage and fewer new splits.
So while the filler will not make existing split ends disappear, it can make the hair look fuller and the ends look better, because the fibre itself is denser and the cuticle is sealed. For long hair with thinning or split ends, this is exactly the kind of product that helps the ends hold up better between trims.
For best results, combine it with a gentle shampoo and a nourishing mask once a week, and keep up with regular trims to remove the ends that are already split.