Yeah, it's old lore from the game's former writer Ivaylo, not reflected in the game proper. I'm not sure if it can even be considered canon anymore.
I'd argue that it can't really be considered canon anymore. For starters, as far as I know the Mastery Epoch is no longer referenced anywhere within the game, and searching descriptions.csv for 'mastery' didn't turn anything up. Then you have the description of the Hammerhead making things somewhat blurry, as it would be redundant, though not wrong, to describe the vessel as a 'Core Epoch midline destroyer' if Core Epoch implied midline and midline implied Core Epoch. You also have the Sunder, which at least in my opinion is a midline destroyer (visual appearances match midline, armament of mixed ballistics and energy is typical of midline, though the flux capacity and dissipation is perhaps a bit on the high side for a midline ship of its armament and the defenses couple the thin armor of high tech ships with the inefficient shields of low tech ships instead of the more typical midline approach of moderately efficient shields coupled with average armor), but which is explicitly stated to be an (early) Expansion Epoch design; if the Sunder is midline, then midline very definitely cannot imply Core Epoch, though Core Epoch could still imply midline (but, of course, the Gryphon and Heron cast doubt upon that).
But for example the Gryphon, which is visually a midline design, is now described as being used just before the Gates collapsed, which would put it in the Expansion-epoch. I don't know if that's a mistake or if David is throwing those old concepts over board.
The Heron is another example of this; it is by appearances, stats, system (midline drones like the Gemini or Atlas rather than high tech drones like the Apogee, Astral, or Tempest), and armament a midline ship, but as the embodiment of a doctrinal shift which was interrupted by the Collapse it'd be somewhat odd for it to be a Core Epoch design since the Collapse ended the Expansion Epoch.
The Monitor is another ship that casts doubt upon the Epoch-tech equivalency, as it is a more-or-less midline design whose description can be read in a way that implies it to be a post-Collapse and thus post-Expansion Epoch design.