Hey CH do hope your doin' well -

I think this has been brought up before, but I sure couldn't say where or when so what the heck . . .
I've always thought Ennis did a remarkably fast think here, coming up with the "fishin' buddies" line. Where'd that come from? Maybe he had other real fishin' buddies, so this was just a plausible add-on to an existing line of friends and buddy events. But we never see anybody actually fishin' (either in '63 or later), so it does seem to be a remarkable bit of creativity on Ennis' part.
But more to your point, why the story to begin with? I'd tend to agree with most of your hypothesis, that Ennis didn't want to go down the slippery slope of the summer of '63 "up on Brokeback" with Alma. There was so much to say about it, and he obviously hadn't said a word to her about it in four years (that says quite a bit about their relationship right there to me anyway).
Moving to the SS for a moment, after Alma spies their passionate embrace
"pressing chest and groin and thigh and leg together, treading on each other's toes" and Annie conveys his conundrum -
"What could he say?" - What indeed, (Ennis was never good with words). So much had happened that summer that had transformed at least a part (even if a hidden part) of Ennis's soul, it would seem that maybe he found it impossible to summarize who Jack was in that instant (or anytime later). Maybe he'd never really figured it out for himself, who, or what, was Jack Twist to Ennis? So how could he describe him now without getting all twisted around in other lies and missteps? So as he'd been taught with his brother K.E.
"Don't say nothin' and get it over with quick". So, as we see, not only does he quickly make up this line, but he then quickly exits the room. End of conversation, huh?
Another small point, but one that I feel is important, "first lie" - it seems we rarely, if ever, see Ennis actually lie. The "fishin' buddy" may only be a small part of the truth, but it is plausible that they actually were fishin' buddies, havin' fished up on the mountain - if only once to improve the "food situation". I think Ennis felt a strong connection with the 'cowboy code', of which one element is to never lie. Were there actual untruths that he told Alma? Partial truths no doubt.