Do self-growing hydrogels grow and strengthen infinitely?
"Strength" is a dubious quantity. The E-modulus will rise, and as the material swells with more and more monomer diffusing in, it will at some point rupture when the brittle network cracks. Like a fruit gum that was put in water overnight.

I have no idea if the two networks actually stay separated, or if they interconnect more and more. I suspect the latter, with radicals at work. That would make the brittle part softer -> no further hardening, and the soft network more brittle -> it snaps under increasing load.

But the point is moot imho: "Self-healing" makes sense if you had a smaller damage due to light overload. Then this material here can become twice as hard, and you're fine. I don't think it makes sense to "train" sth to become fifty times as strong.