Yllendthyr is aware of tank technology and has something kind of like it.
But mithril frames aren't that well suited for heavy cannons, and Yllendthyr's military doctrine focuses more on speed.
Baically they try to rely on alternatives to deal with tanks, including divebombing tactical fighter/bomber craft. WW1 never happened here so nations worldwide were not forced to acknowledge the tactical gains by air superiority. Still, Yllendthyr has a long history on using aerial forces to turn the tide of battle so they try to count on their air forces the same way.

Another interesting thing is that Yllendthyr has no heavy bomber aircraft. They scorn at the idea of making their propellercraft slow. Instead they delegate that role to their airships. Airships would be practically like mobile aerial bases to the elven warmachine. They are sorta like aircraft carriers. Shooting down Yllendthyr's airships can be quite an issue for them. Back to topic, heavy bombings are performed in a rather unique way by Yllendthyr. Their airships usually don't drop bombs but launch V1 like missiles at distant targets.
They use their fast aircraft to scout out the area, relay the coordinates and then launch the missile. It works reasonably well, albeit these "flying bombs" are susceptible to interception so air superiority is advisable.