Cosgrove was handed a two-game ban after being sent off in the closing stages of Auckland's 1-0 FIFA Intercontinental Cup loss to Saudi side Al-Ahli in August.
Auckland are allowed to play A-League professionals in their reserves side under New Zealand Football's participation agreement and competition regulations.
That includes up to two 23-year-old non-citizens for each match day squad.
The clause opened the door for the 29-year-old Englishman to serve out his ban in Auckland's recent New Zealand National League fixtures against Wellington Olympic and Miramar Rangers.
In a fiery exchange on Thursday, Cosgrove couldn't recall the two matches and was bemused when asked if he had any unease or embarrassment over using the "loophole".
"Embarrassment," he shot back at a reporter.
"Why would the club not push to have a player available and use every power that they have?"
Cosgrove bagged 11 goals in his debut season with Auckland, securing the men's competition golden boot along the way to the club's first A-League championship title.
"It doesn't matter, does it? I've missed two NPL matches," Cosgrove said of not serving his ban at the elite level.
"If you pick up a suspension in the A-League, you have to serve it in the A-League.
"So technically, in that case, I should be missing the next few Intercontinental Cups, but I'm not."
The media conference ended abruptly before the reporter repeated his claim that Cosgrove hadn't actually served the ban.
"What are you doing here then? Bugger off, if you're going to talk shit," the striker replied.
An Auckland FC minder stated that FIFA, Football Australia and Football New Zealand were "fine" with the way the ban was served.
"You know what football's like in this country mate. We f**king work hard to get it good," Cosgrove said.
"You bring it down with shit like that.
"What are you laughing for? We'll just f**k you off. You won't get back here again."
In a podcast episode posted on Tuesday, Cosgrove said he was relieved when news of the rules work-around became public so he could "finally stop lying about it".
"It was a poor challenge in oppressive heat and late in the game but the two-game suspension was a little bit harsh," he told The Unused Subs podcast.
"We were trying to get that reduced, and the power of lawyers and legal teams these days they managed to find a way to sign me up with the Auckland reserves.
"It was a good experience to be involved in that league and watch those games."
The defending champions will begin their A-League campaign at home to Melbourne City on Saturday, before an away trip to face Western Sydney Wanderers on October 24.
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