Is this interaction legit?

So here's the story. I just played an ARAM and i got Nunu. I played him as tank and i got "Stoneborn Pact", since we had 2 ADC's. After i got Thornmail i noticed that every time someone auto-attacked me, the stoneborn pact triggered on them. But, in its description says "your movement impairing effects brand enemy champions with an earthen rune" and the cold steel passive from thornmail says "when hit by basic attacks, reduces the attacker's Attack Speed by 15% for 1 second", which is not a movement impairment effect. So im wondering, is this a legit interaction or not?
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