OK, I have looked into the book. Said quote is at the end of the first part of the chapter about pole weapons, p.150. The part is a brief overview of which masters taught what about these weapons. The idea exposed is that pole weapons in general were given far less attention than swords in all traditions, with fewer theories developped about their use.
The paragraph containing the quote underlines that even though pole weapons used in formations (halberds then pikes by the Swiss first, then the Spanish and the Germans) enjoyed increasing success on the battlefield during the period, masters wrote rather about duel applications than fight in formation. Mr Anglo takes the poleaxe as an example of this tendancy. There are three footnotes in the quote that give some sources he used:
Outside the confines of the lists, the poleaxe was not an effective weapon, for, unlike two-hand and bastard swords, they are not well balanced. Their insuffiency is attested by the many acounts of fifteenth-century combats in which knights battered each other unmercifully - denting, puncturing and even knocking bits off each others armour, yet eventually emerging unscathed and often not even out of breath [1]. This suggests that armour was effective, and that axes were not [2]. But they did offer knights scope for Brilliant display, as in a famous combat between Portuguese and French knights in 1415; and some of the early masters of arms also found it a sympathetic subject [3].
[1] For an excellent survey of foot combat within the lists, see Claude Gaier, `Technique des combats singuliers d'après les auteurs "bourguignons" du XVe siècle',
Le Moyen Age, No 3-4 (1985), pp. 415-57; No 1 (1986), pp. 5-40. Viscount Dillon `Barriers and foot combats',
Archeological Journal, LXI (1904), pp. 276-308, is informative but chaotic.
[2] There are exceptions such as the fight between the Earl of Warwick and Pandolfo Malatesta when the latter is supposed to have been `sore wounded' in the left shoulder. See
Pageant of the Birth Life and Death of Richard Beauchamp Earl of Warwick KG 1389-1439, ed. Viscount Dillon and W.H. St John Hope (1914), Plate XIV.
[3] The combat between three Portuguese and three French knights is mentioned by Monstrelet (I, 134) and more fully described by Saint-Remy,
Chronique, ed. F. Morand (Paris, 1876-81), I, pp. 208-10.
That's about as far as I can get as I have none of the sources mentioned
Maybe the length of the weapon itself has something to do with the assessment. It seems to me that this is a difference between a poleaxe and a halberd. The poleaxe seems to be some intermediate between a warhammer and a halberd, both successful battlefield weapons, but maybe the middle ground in this case is not such a good choice? Edit: Actually I'm not even sure about the warhammer here
Hope this helps!