First let me say that I don't own this car nor have I ever seen the underside of one. What I've found is purely through online research.
To hopefully answer your question, yes that part should be responsible for adjusting toe. Mitsubishi calls it a lateral arm. Unfortunately that part is a stamped steel arm with no elongation adjustment. Adjustment seems to come from an eccentric bolt on the side that connects to the subframe.
If you can't get enough toe reduction out of the bolt then your next bet would be to find an aftermarket adjustable arm. Unfortunately I wasn't able to find anyone that makes a set of those. You might have to just have a welder shorten the arms you already have to bring the original adjustment back into range. If you've got excessive toe out then you'll need to shorten not lengthen.