They that speak in proverbs - The original word is almost equivalent to "the poets." The word supplies the title of the Book of Proverbs itself; and is used of the parable proper in Ezekiel 17:2; of the prophecies of Balsam in Numbers 23:7-10; Numbers 24:3-9; etc.; and of a song of triumph over Babylon in Isaiah 14:4.

They that speak in proverbs - המשלים hammoshelim, from משל mashal, to rule, to exercise authority; hence a weighty proverbial saying, because admitted as an axiom for the government of life. The moshelim of the ancient Asiatics were the same, in all probability, as the Poetae among the Greeks and Latins, the shaara among the Arabs, who were esteemed as Divine persons, and who had their name from shaara, he knew, understood; whose poems celebrated past transactions, and especially those which concerned the military history of their nation. These poets were also termed sahebi deewan, companions or lords of the council of state, because their weighty sayings and universal knowledge were held in the highest repute. Similar to these were the bards among the ancient Druids, and the Sennachies among the ancient Celtic inhabitants of these nations.
The ode from the 27th to the 30th verse is composed of three parts. The first takes in Numbers 21:27 and Numbers 21:28; the second Numbers 21:29; and the third Numbers 21:30.
The first records with bitter irony the late insults of Sihon and his subjects over the conquered Moabites.
The second expresses the compassion of the Israelites over the desolations of Moab, with a bitter sarcasm against their god Chemosh, who had abandoned his votaries in their distress, or was not able to rescue them out of the hands of their enemies.
The third sets forth the revenge taken by Israel upon the whole country of Sihon, from Heshbon to Dibon, and from Nophah even to Medeba. See Isaiah 15:1, Isaiah 15:2.
The whole poem, divided into its proper hemistichs, as it stands in Kennicott's Hebrew Bible, is as follows: -
Verse 27. Part I
Come ye to Heshbon, let it be rebuilt;
The city of Sihon, let it be established.
Verse 28
For from Heshbon the fire went out,
And a flame from the city of Sihon:
It hath consumed the city of Moab,
With the lords of the heights of Arnon.
Verse 29. Part 2
Alas for thee, O Moab!
Thou hast perished, O people of Chemosh!
He hath given up his fugitive sons
And his daughters into captivity,
To the king of the Amorites, Sihon.
Verse 30. Part 3
But on them have We lifted destruction,
From Heshbon even to Dibon;
We have destroyed even to Nophah,
The fire did reach to Medebah.
See Kennicott's Remarks.

Wherefore they that speak in proverbs say,.... The historical writers of those times, among the Amorites, who were usually poets, and wrote the history of the wars between the Moabites and Amorites in verse; as Homer among the Greeks wrote the wars of Troy; and the compositions of those ancient bards were short and compendious, and wrapped up in proverbial sayings, and enigmatical and figurative expressions, that they might be the better retained in memory, and therefore were called proverbialists. Jarchi says, they were Balaam and Beor that took up their parables, and said:
come into Heshbon; which words are the beginning of the song, and in which the Amorites are represented as inviting Sihon, and his nobles, to enter Heshbon, which he had taken, and make it his royal seat; or as encouraging one another to go into it and repair it, having suffered much at the taking of it, which seems to be confirmed by what follows:
let the city of Sihon be built and prepared; that is, let us set about rebuilding of the city, and let us fit it up for Sihon our king, and let it be called his city, and made the place of his residence, his palace, and where his court may be kept.

Wherefore they that speak in proverbs--Here is given an extract from an Amorite song exultingly anticipating an extension of their conquests to Arnon. The quotation from the poem of the Amorite bard ends at Numbers 21:28. The two following verses appear to be the strains in which the Israelites expose the impotence of the usurpers.

In proverbs - The poets or other ingenious persons, of the Amorites or Canaanites, who made this following song of triumph over the vanquished Moabites: which is here brought in, as a proof that this was now Sihon's land, and as an evidence of the just judgment of God in spoiling the spoilers, and subduing those who insulted over their conquered enemies. Come into Heshbon - These are the words either of Sihon speaking to his people, or of the people exhorting one another to come and possess the city which they had taken. Of Sihon - That which once was the royal city of the king of Moab, but now is the city of Sihon.