Not a tender steak

You must be in Canada. The Canadian beef grading system is as follows, and these are all considered in the category of "high quality":

Prime - abundant marbling, about 2 percent of beef is graded as prime

AAA - small amounts of visual marbling, very high quality, up to 50% is graded AAA

AA - slight amount of marbling, a step down from AAA, 45% of beef is graded AA

A - the lowest rating, up to 3% of Canadian beef is rated A. Less evenly distributed fat...needs more attention and care when cooking.

Given the AA rating, you probably did not have much fat/marbling to help the texture.

One suggestion is to employ sous vide cooking. With this approach, you can make cuts more tender by cooking them longer, but also retaining the desired level of doneness.

I agree with part of moscafj's answer in regard to grading: the lack of marbling may have contributed to dryness as well as some reduction in tenderness.

However, I'm not sure that I agree that a strip loin steak is "not the most tender cut of meat to begin with." It's cut from the back part of the longissimus dorsi muscle (a muscle that receives very little work, and is thus tender), the part that makes the ribeye segments in the front of the cow that dreams are made of. Yes, it's not as tender as a filet mignon/tenderloin or ribeye, but it's difficult to find anything else on the cow that's as tender as a decent strip steak. (So, it probably beats out maybe 90% of the other meat on a cow for tenderness.)

All of that said, I've had very tender great strip steaks, and I've had others that were chewy and dry (as you experienced), even when cooked properly. Just as I've had awful ribeye and porterhouse steaks from some stores. Meat quality can vary a lot depending on your source, and a lower grade won't help.

It's tough to know what else may have gone wrong, but here are a few thoughts:

The question mentions "thawing in the fridge," so I assume this was a frozen steak too. Freezing, particularly when done in the home fridge (rather than rapid commercial freezing) can also remove moisture and have detrimental effects on texture.

Also, I'd note the description of the cooked steak: "It was cooked to a perfectly medium rare, quite pink in the centre with distinctive grill marks." Grill marks may look pretty, but they won't make a steak tender or juicy (and actually can harm these qualities slightly if you go overboard). As for "pinkness," I might recommend checking temperature with a thermometer the next time to see exactly what temperature you're ending up with. Color is not always a reliable indicator of doneness, and sometimes by the time a steak looks "pink" (instead of "red") it's well past the maximum juiciness medium-rare stage.

Lastly, if you're looking for cooking techniques to maximize tenderness and juiciness of steaks, you can't go wrong with moscafj's sous vide suggestion. But if you're looking for something less involved and which may not require specialized equipment, I might look into the so-called "reverse sear" technique, where you heat the steak for a while in a low oven, then just sear quickly at the end on your grill (or in a pan). (It's called "reverse" as it's the opposite of what steakhouses traditionally do, where one sears first and puts into an oven to finish the steak throughout. Both techniques will avoid wider gray bands of overdone and dry meat near the edges, though the "reverse" is better for maximizing tenderness with a longer slow cook at the outset.)

I see nothing about letting the steak rest after cooking, which should be done for at least 5-10 minutes if not more for really thick cuts. If you cut into the steak immediately, all of the juices end up on the cutting board/plate, and not in your mouth.

I think what you might have experienced is - a tissue steak. I frequently buy strip loins, and there's a very specific thing you need to look for when buying them. If there's a prominent 'half circle' of gristle - you're probably dealing with a strip loin that was cut from the less pleasant end of the primal (sirloin end.) They are notoriously chewy and fibrous. For more information see here

For tough cuts of beef, dry brining is a wonderful technique. I also question, as above, if this cut would be considered tough.