I just can't get FeedMe to work
I believe we figured this out over Slack, but the issue here was that you need to actually import your Line Id column into your lineId field, otherwise Feed Me doesn't know anything about that.

The checkbox to tell Feed Me to use a certain field's value to match existing elements on is one half of the problem - you've already correctly chosen Line Id. The issue is you need the other half - what data in your feed is supposed to match up with Line Id and lineId.

I suppose a less confusing UI would be to have an accompanying dropdown field near those checkboxes to say which bit of data in your feed should you be comparing with.