Sugden's concept of singlet linkage
What he said
He did not say that the octet rule is never violated.
On the contrary, he suggested that &quot;the maximum number of electrons in the valency orbit can not [sic] exceed eight&quot;. (source)
This means, while he denies that the octet rule is violated by molecules like chlorine pentafluoride, he did not challenge whether molecules traditionally labeled as octet-deficient such as beryllium difluoride indeed violates the octet rule.
He argued for the existence of a two-center one-electron bond.
Why his theory was poorly accepted
Preconceptions?
The success of his theory
To quote Wikipedia:

In the 1940s and 1950s, Rundle and Pimentel popularized the idea of the three-center four-electron bond, which is essentially the same concept which Sugden attempted to advance decades earlier; the three-center four-electron bond can be alternatively viewed as consisting of two collinear two-center one-electron bonds, with the remaining two nonbonding electrons localized to the ligands.

Indeed, the three-center-four-electron explains the bonding in the triiodide anion well.
Also, another success is that this theory, unlike the expanded-octet theory, does not rule out the existence of hypervalent second-period element, such as carbon (&quot;Frozen&quot; Transition States: Pentavalent Carbon et al., J. C. Martin, Science, 05 Aug 1983, Vol. 221, Issue 4610, pp. 509-514, DOI: 10.1126/science.221.4610.509).
The problems with this theory
He was reluctant to accept that ionic characters do indeed play a larger in some molecules traditionally labelled as hypervalent, such as the sulfate anion.
The failures of this theory
In the sulfate anion, there is no two-center one-electron bond (Wikipedia):

For sulfuric acid, computational analysis (with natural bond orbitals) confirms a clear positive charge on sulfur (theoretically +2.45) and a low 3d occupancy.

Also, some of the traditionally-labeled hypervalent molecules involve very electronegative elements such as fluorine for sulfur hexafluoride, oxygen for sulfate, and chlorine for phosphorus pentachloride. A better explanation for these molecules would need to invoke ionic characters instead of the covalent singlet linkage.
For sulfur hexafluoride (source):

The sulfur d orbitals are found to contribute very strongly to the binding energy ($250$ kcal/mol) and to have a  total occupancy of around $0.25\mathrm{e}$, the valence electron distribution on sulfur being about $32\%$ in $3\mathrm{s}$, $59\%$ in $3\mathrm{p}$, $8\%$ in $3\mathrm{d}$, and $1\%$ in $4\mathrm{p}$. The occupancy of the two sulfur $3\mathrm{d}_{\sigma}$ orbitals ($0.16\mathrm{e}$), however, is only one-sixth of what would be required for $\mathrm{s}\mathrm{p}^3\mathrm{d}^2$ hybridization, and the energetic contribution of these orbitals is only two to three times larger than that of the sulfur $3\mathrm{d}_{\pi}$ orbitals. The sulfur $\mathrm{d}$ orbitals are important because they allow strong back transfer from the negatively charged fluorine ligands to the strongly positively charged ($+2.9\mathrm{e}$) sulfur, in turn allowing significant contraction of the $\ce{S-F}$ bonds and greatly increased molecular stability.