As long as plenty of water is used, the acid dilutes very easily and wouldn't normally cause an issue, however it's not an easy question to answer as it would certainly have an impact on the acidity of the soil.
It's not going to be poisonous to plants directly, however the change in soil chemistry might not be favourable, we recommend avoiding overspill wherever possible.
The Etch'N'Clean is a hydrochloric acid and, as such, would make any soil it comes into contact with acidic. Not all plants thrive in an acidic environment.
Our advice would be to not pour the acid onto any soil or plants. The reaction of the acid with the surface typically neutralises the acid and the recommended scrubbing and rinsing will further dilute any remaining acid.
This should minimise any damage to plants, however, depending on the sensitivity of the plants we cannot say there won’t be any damage. Fortunately though, the damage will not be permanent as the soil will neutralise naturally over time.