Why does the Buddha's harsh speech, in the Pali Canon, is not harsh speech?

Harsh speech follow to Collins dictionary:

Harsh actions or speech are unkind and show no understanding or sympathy.

Wrong speech follow to Sutta. Ma. Mū. Verañjakasuttaṃ:

Would talk roughly, saying insolent words that are sharp and cursing. Words bordering on anger and not conductive to concentration. Would say frivolous, untimely, untruthful words, not in keeping with the Teaching and the discipline.

But the buddha's words is more flexible, beniefical, in Sutta. Ma. Ma. Abhayarājakumārasuttaṃ:

1 In the case of words that the Tathagata knows to be unfactual, untrue, unbeneficial (or: not connected with the goal), unendearing & disagreeable to others, he does not say them.
2 In the case of words that the Tathagata knows to be factual, true, unbeneficial, unendearing & disagreeable to others, he does not say them.
3 In the case of words that the Tathagata knows to be factual, true, beneficial, but unendearing & disagreeable to others, he has a sense of the proper time for saying them.
4 In the case of words that the Tathagata knows to be unfactual, untrue, unbeneficial, but endearing & agreeable to others, he does not say them.
5 In the case of words that the Tathagata knows to be factual, true, unbeneficial, but endearing & agreeable to others, he does not say them.
6 In the case of words that the Tathagata knows to be factual, true, beneficial, and endearing & agreeable to others, he has a sense of the proper time for saying them. Why is that? Because the Tathagata has sympathy for living beings."

So, when buddha blame Sāgata-bhikkhu in VN Surāpāna-sikkhāpada (and the other bhikkhus in many Vinaya rules [sikkhāpada]), it is not harsh speech, because after that, he became an arahanta.
However, in Devadatta case, nobody could change his vindictiveness, so buddha made devadatta as a cause of statute making instead in Vinaya. Mahāvi (1) Saṅghādisesa Dasamasaṅghādisesaṃ. Also, buddha try to keep Devadatta's students instead of him in Vinaya. Culla (2) Saṅghabhedakkhandhakaṃ, so buddha helped Devadatta by helping his students:

the venerable Sāriputta spoke thus to the Lord: “Devadatta, Lord, having split the Order, is setting out for Gayā Head with as many as five hundred monks.”
“Can there not be for you, Sāriputta and Moggallāna, compassion for these newly ordained monks? Go you along, Sāriputta and Moggallāna, before these monks fall into trouble and distress.”