Yes it's "just releasing some chemicals", vast and complex regulatory networks of chemicals.

They then did RNAseq experiments (measures which genes are transcribed [are active]) in the two legs separately and looked for statistically significant differences (which genes are active in the injected leg but not the other one?) and found:

"enrichment of several pathways related to stress and immune responses in the left cartilage.In particular, we found several stress-related transcripts that shared a similar left–right pattern of expression within each embryo"