The ferries now use Cairnryan, but as they are mentioned in the article I guess passengers are - or could be - bussed the 5 miles to Stranraer. The railway doesn't go past there, so there are no options for a station or realistically for freight.
In the past, Cairnryan was rail served (from Wikpedia):
During World War II, Cairnryan became No.2 Military Port, and three harbour piers and a military railway, linking the village with nearby Stranraer, were built by the army. For a period after the war, continuing at least until 1958, the port was used to receive, by rail and Liberty/Victory ships, surplus/time-expired ammunition which was loaded onto army landing craft for disposal at sea. That coming by rail, had trucks labelled with the address, "Davy Jones' Locker, Cairnryan".
But now the transfer is from Ayr by bus / which according to some reports has become a minibus.
You're spot on with your comments about the dangers of stock photography, and the apparent issues with running bimode trains to Stranraer. The line, with the ferry connection gone, becomes one of the more difficult cases around to run cost effectively (that's written with little local understanding of the area - I stand to be corrected) and the station use figures for Stranraer show either some really good marketing to keep the numbers up when the ferry left, or ferry service that in practise took very few people arriving there by train even when it left from the Harbour station.
Annual journeys:
Stranraer around 45,000
Barrhill around 5,000
Girvan around 140,000 (some trains turn back there)
Maybole around 70,000
And so to Ayr. Adding the above, making a small allowance for local traffic, you've got around 500,000 journeys a year on about 26 trains a day - something like 50 passengers per train, which ain't bad. Taking the section south from Girvan, 12 trains per day, you've got figures of around 23 passengers per train. So three quarters of the passengers use the northern third of the line (measured in journey time) and only a quarter use the southern two thirds.