The difficulty is that until Book 9 drops, we don't actually know the full detail of what the Dark Angels Legion actually is, other than what we've been able to glean from the novels, because we don't have a Black Book to tell us what they are or were. As much as we might nitpick or disagree with their content, the Black Books are gospel, and that's just that.
Besides which, Terminators and Bikes, i.e the Deathwing and the Ravenwing, came to the forefront of the Dark Angels AFTER the Fall. The Fall was obviously a cataclysmic event for the I Legion, and the shame of it has echoed through the millennia. They shifted their entire modus operandi and culture in order to kill or capture the fallen so as to wipe the stain from their honour.
And that's the key. They shifted. They changed. To expect a focus on Terminators and Bikes is too limiting because the I Legion weren't desperately hiding the secret that killed their Primarch yet.
I love the idea of the First as a testbed Legion, because the Lion strikes me as a 'by any means' type of warlord. He's 100%, purely loyal to the Emperor, and is not at all concerned about the opinion or the approval of others. He's playing to an audience of one, and if the Emperor approves, that's enough. 'Loyalty is its on reward', after all. It makes sense, in my head, for the Lion and his Legion to make use of whatever weapons or wargear might get the job done, regardless of those method's palatability to the other legions.
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