When the tnt is burned you can push it using any sword with knockback enchantment
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because it's an entity

Yes but its only at 21w18a version and at earlier version it doesn't work
Can confirm. This issue only seems to exist in 21w18a as stated above already, however, this makes sense as primed TNT is considered an entity.

Also works with a knockback attack by sprinting. Relates to MC-175132.