My point is calling it more then a guess is overselling it. You got three shirt hints featuring the Grim Reaper on it, one of which doesn't even have a scythe on it, and them being coy about revealing Zenos's new weapon. No more, no less.
And from that people are guessing the shirts mean a scythe wielding class and Zenos is going to showcase this new class. What happens if the shirt hints reference a class with death themed armor and death themed abilities and no scythe? That's a very real possibility.
And I remember when the first trailer for Stormblood dropped and a lot of people jumped to the assumption that Lyse sparing with the WoL atop that statue was a fight between a Monk and Dancer, that the Girl In Red (Lyse) was a Dancer and Dancer was going to be a job added in Stormblood. Turned out it was just two Monks sparing. That alone should serve as a warning about taking something ambiguous from a trailer as a near certainty.
Yoshida's 007 shirt had people guessing Agent (which was a job in Tactics), Musketeer, Gunner; very few people guessed Machinist. Most people agreed it was a gun using class but that was about it. The Spiderman shirt had a lot of people guessing Blue Mage - because Peter Parker got bitten to get his powers and a Blue Mage gets attacked by monsters for its powers - and a lot of people mocked the Sam Raimi / Samurai theory. You are looking at these shirt hints with the benefit of hindsight and it is that hindsight make it seem like they have never been ambiguous but looking back on the discussion threads at the time paints a very different picture.
And Hades is both the Greek God of the Underworld and of the Dead. I think it is disingenuous to say that the name is not connected with death because the Greeks had other gods associated with death like Thanatos and Charon. Also, saying the fact the Garlean Army fields an armored vehicle called the Reaper is not evidence of a potential Garlean soldier class because we fight on foot is in the same category as saying the scythe can't be a weapon because the Botanists use it already in game.
And yes, we can reasonably assume that we will be dealing with the Garlean Army for a good chunk of Endwalker from 5.5. We know they are building Zenos's tower and will likely be garrisoning it and the area around it. We know a large chunk of the Garlean Army have joined up with the Telophoroi. We know they released dungeon artwork showing a train depot at a Garlean military base. We even had Fandaniel bragging that the Garlean Empire and Army was the cornerstone of his plan in the last cutscene: "A mighty Empire, now no more then an instrument for this star's destruction. What a pleasure it will be to put it to use!"