The stanza OP is querying, from Dryden’s imitation of the Second Epode of Horace (The Hymn of Gentry Contentment) is:
How happy in his low degree,
How rich in humble poverty, is he,
Who leads a quiet country life;
Discharg’d of business, void of strife,
This almost scans as iambic tetrameter: 8–9 syllables, not 10–11 syllables (which the rest of the poem does, outside its final stanza), and it scans better than OP thought it did:
1. In modern times, qui.et is two syllables, not one, although past spellings reveal that it used to be both. From OED 3rd edition:
ME quit, ME quyeet, ME quyte, ME qwiet, ME qwiete, ME qwyete, ME–15 quiete, ME–15 quyete, ME–16 quyet, ME– quiet, 15 quiate, 15–16 quiett, 15–16 quyett, 16 queat, 16 queit, 18– quate (south-west midl. and Irish English (lnorth.)), 18– quite (Lancs.), 19– quait (Irish English (north.)), 19– quayit (Eng. regional (Devon)), 19– quient (U.S. regional); Sc. pre-17 quayt, pre-17 queat, pre-17 queet, pre-17 queyt, pre-17 queytt, pre-17 quiatt, pre-17 quiete, pre-17 quiett, pre-17 quiette, pre-17 quiit, pre-17 quit, pre-17 quoyet, pre-17 quoyit, pre-17 quyat, pre-17 quyatt, pre-17 quyet, pre-17 quyete, pre-17 quyett, pre-17 quyiat, pre-17 quyiet, pre-17 quyit, pre-17 quyt, pre-17 qwiet, pre-17 qwiette, pre-17 qwyet, pre-17 qwyete, pre-17 qwyett, pre-17 qwyette, pre-17 qyett, pre-17 17– quait, pre-17 17– quiet, pre-17 19– queyet, 18 quaete, 18 quaiet, 18 quayet, 18– quaite, 18– quate, 19– quaeit, 19– quaet, 19– quaiat, 19– quet, 19– queyit, 19– qui’t, 19– quite.
2. In modern times, business is two syllables (bɪznəs). To quote OED again:
Disyllabic pronunciation, reflecting syncope of the unstressed second syllable of trisyllabic forms, is indicated by spellings without a medial vowel from the 16th cent. and is noted by orthoepists from the early 17th cent. (see further E. J. Dobson Eng. Pronunc. 1500–1700 (ed. 2, 1968) II. §306).
3. However, poverty is a problem. For this to scan, it would have to be one of:
- pov’rty’s he (with pov’rty two syllables, and not <r> acting as its own syllable). But that’s unpronouncable, unless the <v> was actually a /w/. OED provides a pre-17th century spelling powerte, powertie, and pow’rty’s he is slightly more pronounceable; but that was supposed to have been ancient history by Dryden’s time.
- povert’s he, with the final -y not pronounced; OED indicated that that did actually occur in places in Middle English and Modern dialect—Middle English has spellings like “ME pouerd, ME pouerert (transmission error), ME pouerte, ME pouertt, ME pouertte, ME povert, ME poverte, ME powaret, ME–15 pouert; Sc. pre-17 powert.” Again, that was supposed to have been ancient history by Dryden’s time.
… There is of course a simpler explanation: that Dryden is allowing himself one initial pentameter, before ploughing on in tetrameters; just as he puts a couple of trimeters at the very end of the poem—
This Morecraft said within himself,
Resolved to leave the wicked town,
And live retired upon his own.
He called his money in;
But the prevailing love of pelf
Soon split him on the former shelf,
And put it out again.
I don’t know enough about English verse to know whether such licence was commonplace at the start of long poems.