The paper[1] referenced by Nilay Ghosh says that,
it is well known that hydrogen bonds commonly exist in ammonia clusters and play an important role
and gives a picture of some of the clusters that were assumed to exist. So there seems to be evidence of significant covalent hydrogen bonding in possible ammonia clusters, at least from the calculations.
Calculated ammonia clusters:
Whereas the reference[2] given by the OP was less positive:
Surprisingly, no evidence has been found to support the view that $\ce{NH3}$ acts as a proton donor through hydrogen bonding,
but does describe ammonia as a powerful hydrogen-bond acceptor.
There is paper[3] which describes a hydrogen-bonded dimer that demonstrates how well ammonia can hydrogen-bond as an electron donor.
Spectrum of hydrogen-bonded dimer of ammonia:
But the question remains: "If ammonia is as basic, relative to water, as acetic acid is acidic, why the very different behavior?" According to a journal article[4], acetic acid is known to be significantly dimeric in the vapor phase: 50% of the vapor is dimer at $\pu{1 atm}$ and $\pu{400 K}$.
Compressibility factor vs Temperature for saturated acetic acid vapours:
One simple way to look at the difference is that according to chem.ucla.edu[5], hydrogen has an electronegativity of 2.1, nitrogen 3.0, and oxygen 3.5. The difference between the electronegativities of $\ce{H}$ and $\ce{N}$ generates a small protonic acidity. According to Chem LibreTexts[6] $\mathrm{p}K_\mathrm{a}$ of $\ce{NH3}$ is 34. Very small!
The difference in electronegativities between $\ce{H}$ and $\ce{O}$ is larger, and gives, for hydroxy compounds like $\ce{H2O}$ and $\ce{CH3OH}$, $\mathrm{p}K_\mathrm{a}$ values which are in something on the order of 14-15. So, while both $\ce{N}$ and $\ce{O}$ have lone pairs that are electron-rich, the polarity of the protons on nitrogen is not great enough to develop strong - polar - hydrogen bonding.
I suspect that covalent bonding was suggested in [1] as a means to bond the structures with molecular orbitals. Whether these structures or the calculations play a significant role in the behavior of ammonia is unclear.
Getting back to the quote by the OP, I'm not impressed with the claim that ammonia shows much in the way of association by means of its melting and boiling points:
$$
\begin{array}{|c|c|c|}
\hline
\text{Compound} & \text{Melting Point} & \text{Boiling Point}\\
\hline
\ce{CH4} & \pu{-182 ^\circ C} & \pu{-161 ^\circ C}\\
\hline
\ce{NH3} & \pu{-78 ^\circ C} & \pu{-33 ^\circ C}\\
\hline
\ce{H2O} & \pu{0 ^\circ C} & \pu{100 ^\circ C}\\
\hline
\end{array}
$$
Ammonia lies pretty much in between methane, with no hydrogen-bonding capability and water, with considerable hydrogen-bonding capability. If ammonia were to be considered to be capable of hydrogen-bonding (i.e., dimerization) in the vapor, then water should be even more so. The water dimer has been investigated[7] and values of bonding are suggested to be between $\pu{5-6 kcal/mol}$, but the proposed structure does not suggest any special stability:
Perhaps we could be more interested in water association!
References:
[1]: Wang, B.; Hou, P.; Cai, Y.; Guo, Z.; Han, D.; Gao, Y.; Zhao, L. Understanding the Hydrogen-Bonded Clusters of Ammonia (NH3) n (n = 3-6): Insights from the Electronic Structure Theory. ACS Omega 2020, 5 (49), 31724–31729.
[2]: Nelson, D. D., Jr; Fraser, G. T.; Klemperer, W. Does Ammonia Hydrogen Bond? Science 1987, 238 (4834), 1670–1674.
[3]: Wang, H.; Agmon, N. Complete Assignment of the Infrared Spectrum of the Gas-Phase Protonated Ammonia Dimer. J. Phys. Chem. A 2016, 120 (19), 3117–3135.
[4]: Tsivintzelis, I.; Kontogeorgis, G. M.; Panayiotou, C. Dimerization of Carboxylic Acids: An Equation of State Approach. J. Phys. Chem. B 2017, 121 (9), 2153–2163.
[5]: Illustrated glossary of organic chemistry - electronegativity
http://www.chem.ucla.edu/~harding/IGOC/E/electronegativity.html
(accessed Jun 27, 2021).
[6]: Libretexts. 21.4: Acidity and Basicity of Amines
https://chem.libretexts.org/@go/page/32545
(accessed Jun 27, 2021).
[7]: Wikipedia contributors. Water dimer
https://en.wikipedia.org/w/index.php?title=Water_dimer&oldid=996228917
(accessed Jun 27, 2021).
