If the debate is about the effectiveness of Rifles, it is interesting to look at the first two quotes in their original form, rather than the heavily edited versions found in Gates' work.
The full Bunbury quote is:
"The enemy from the crest of the sand-dunes kept up a constant and destructive fire, while he was himself sheltered by their folds from the guns of the British shipping. None of our field artillery was yet landed, and for some hours our brave infantry, though reinforced by D'Oyley's brigade of Guards, had to maintain a trying struggle; but they would not give ground; and they fought so hard that they fairly wore out the enemy. Towards the evening the Dutch gave up the contest, and retreated some five or six miles to a position between the Alkmaar canal and the sea. The loss of the enemy in killed or wounded was probably small; but the disadvantages under which the invaders fought necessarily exposed the three brigades engaged to a serious loss of men; and a large proportion of the superior officers of the staff fell under the aim of the Dutch riflemen...."
In case it is doubted that the defenders were Dutch and that at least some of them were armed with rifles, the following can be found in The History of the Honourable Artillery Company:
"Serjeant James Milne, of the 3rd regiment of Guards, being introduced into the court, begged leave to present to the colonel a rifle barrelled musket, which in the late expedition to Holland he, on the 27th August last, when the British army landed near the Helder, took from a Dutch rifleman; and with which rifle he, Serjeant Milne, at the battle of Langdike, shot a French general of brigade."
So this does seem to indicate that it was the accuracy of rifled arms that was responsible for the large amount of casulaties amongst superior officers.
The full passage by Surtees, who was serving at the time with a musket-armed composite light battalion, also confirms that he is referring to rifled arms and not just using a generic term for skirmishers:
"we soon got into a smart fire from the enemy's riflemen, which we found was the only description of troops, except a few artillery, that we had to contend with, their main bodies of heavy infantry being on the right and left of this sandy range, which in some places was about a mile in breadth, in others more or less. After the fight had fairly commenced, we kept but little order, owing partly to the want of discipline and experience in our people, and partly to the nature of the ground, which was rugged and uneven in the extreme, being one continued range of sand-hills, with hollows more or less deep between them; and partly it may be attroibuted tio the ardour of our young men, who pressed on perhaps too rapidly. We continued to advance, and never once made a retrograde movement, the enemy regularly retiring from height to height on our approach; but they had greatly the advantage over us in point of shooting, their balls doing more execution than ours; indeed it cannot be wondered at, for they were all riflemen, trained to fire with precision, and armed with weapons which seldom fails its object if truly pointed; while we werer totally ignorant of that most essential part of a soldier's duty."
Perhaps these troops were also Dutch and not French? The Dutch rifles, if surviving examples in the UK are anything to go by, were very similar to the later Baker Rifle and no doubt had a large influence on its design. It would be interesting to know how many of the Dutch Jagers were rifle-armed in 1799.