How did Rama kill Rakshasa Viradha even though the latter had a boon that no weapons can harm him?

Question

The below quotes are taken from Bibek Debroy's English translation of Valmiki Ramayana.

In this world, I would not be slain by any weapon, nor would it be able to penetrate me or slice through me. Cast aside this woman. Forget about her and go wherever you came from.

Here Viradha says that he cannot be killed or harmed by any kind of weapon. But in the next Paragraph, we see that Rama's arrow penetrated Viradha's body and he fell down to the ground.

Tufted with peacock feathers and like fire, they penetrated Viradha’s body and smeared with blood, fell down on the ground.

Later both Rama and Lakshmana severed the arms of Viradha with their arrows.

Soumitri severed the left arm of that terrible one. Rama swiftly severed the rakshasa’s right arm. With his arms severed, the rakshasa quickly fainted and fell down on the ground like a cloud, resembling a mountain shattered by the vajra.

If Viradha had a boon that no weapon can harm him, then how did Rama and Lakshmana severed the arms of Viradha with their arrows?

Answer 1

Viradha's body part couldn't be cut by any weapon. If you read next paragraphs and chapters then, you will find that his body parts got joined again after Rama cut them.

At last, Rama ordered Lakshmana to dig a pit and bury the demon as he couldn't be concurred by any weapon:

"Oh! Manly-tiger Lakshmana, it is impossible to overpower this demon with weapons in a confrontation, hence, let's bury this demon... 3-4-10

"Lakshmana, a very deep pit be dug for this furious and ferocious reprobate in this forest, as though for an elephant... 3-4-11

On saying to Lakshmana that a trench shall be dug...' then Rama repressively placed his foot on the throat of Viradha, and stood by. 3-4-12

Actually, Viradha was a Gandharava but due to a curse attained demon body:

On hearing that said by Raghava, demon Viradha sincerely spoke these humble words to Rama, the finest one from Kakutstha dynasty. 3-4-13

"Dead I am, oh! Manly-tiger and a coequal of Indra in your strength... I have fallaciously not identified you earlier... oh, best one among men... 3-4-14

"By curse I had to enter this ghoulish demon's body, but I am a celestial gandharva, named Tumburu and Kubera cursed me... 3-4-16

"When entreated by me, he that greatly renowned Kubera said to me, 'When Dasharatha's Rama kills you in a fight... then you will attain your nature of celestial body and you will go to heavens...' 3-4-17, 18a

Also, burying was custom for demons as said by Viradha:

"Rama you go safely on burying me in a pit... this is the age-old custom for those demons that lost vitality... those that are inhumed in pit, to them there will be the worlds of manes..." 3-4-22, 23a

And finally Viradha was buried in a pit:

On listening to that which is said by Viradha, Rama resolved his mind to cast him into pit, and when that mighty demon is being flung into the pit the whole forest blared with that demon's cries. 3-4-32