Yes but it's basically coming off (and you said it explicitly) you're not getting the answer you want to hear here.
So in order:
No, no fluff is ignored. You're simple being unwilling to accept more modern fluff over older fluff originating from Space Marine 1. The falchions and shadowsword tanks have been retcon'd as being in fact different designs of tanks by Forgeworld when they made the HH series. FW did provide a bit of fluff to ease the newly envisioned legion falchion as the term falchion was often used generically by the Imperial Army as referring to any dedicated titan hunter. Not perfect but it was least something for those that have played the older epic games.
Anyways, each list is based on a theme and specifically for Steel Legion that theme is mechanized infantry not relic war engines. There's little need in the SL tournament list for an up gunned Shadowsword. I'll point out that player just took a win in the highly competitive UK scene with a 12 unit shadowsword army. A falchion simply isn't needed to make a competitive list more competitive.
The SL list is basically the representative list for the prototypical mech guard regiment found throughout imperial space and raised en masse on a million worlds. Relic war engines are simply anything but typical when viewed from that perspective.
All the super heavy tanks ARE present in the lists, just not in the same one. All those SHTs for instance can be found in the Tank Legion list where they make thematic sense (and easier to balance). The two that don't make an appearance are due to the inability of the EA system to distinguish them as they basically becomes the same stats so why bother?
Titans are a poor choice from a thematic perspective as a comparison for numerous reasons. Foremost is that titans are the absolute religious avatar for the Mechanicum and every Forgeworld aims to build, maintain, and field them. The falchion and glaive tanks are specifically mentioned as utilizing materials and techniques that were little understood and designed for the unique requirements of the legions and giant 7foot tall super humans in space amrour
Now a relic list sounds kinda fun and might be a fun thing to have. Perhaps it's a great candidate s an upgrade to the Heavy Mech list?
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