As the questioner has observed, the 1907 Imperial Conference was primarily about Britain's official colonial holdings and the question of Zionism and Arab nationalism never arose. Before this translated essay was published in 2007, I could only find one source that connects Campbell-Bannerman directly with the Middle East: a 1985 U. Texas-Austin dissertation, "Iran and the Big Powers 1900-1953", where Campbell-Bannerman is blamed for being disinterested in the Middle East:
The Anglo-Russian domination of Iran further lapsed with a change of
government in Britain from the Conservative leadership of Lord
Salisbury to a Liberal government under Sir Henry Campbell-Bannerman.
At a critical time, these British and Russian preoccupations allowed
the Iranian nationalists to consolidate their movement against foreign
interference in their domestic affairs. (ibid, p. 11)
The 1907 Imperial Conference was a decidedly pro-local autonomy conference, at which the British colonies were deemed Dominions. The 1911 book The Imperial Conference; a history and study says:
...there was no longer in Britain any party whose Imperialism followed
the maxim divide et impera.
So it appears that there was no statement like this offered at the 1907 Imperial Conference, and it would have been an uncharacteristic thing for Campbell-Bannerman to endorse.
Was any statement like this possible from any European at such an early date? Other period sources make such a claim sound quite dubious. According to Yaacov Ro'i's article "The Zionist attitude to the Arabs 1908–1914" (Middle Eastern Studies 4.3, 1968), there was little thought about Arab presence in Palestine before 1908, and for the first decade afterwards, what was written about Arab nationalism was often idealistic. One "highly confidential" memorandum in 1913 stated:
It is not possible to ascertain exactly how organized and strong the
Arab movement is yet. Even if it does not seem strong at present, its
future development cannot be anticipated. We must, of course, take
this movement into account much more than do the Turks ...
In our opinion an agreement with the Arabs on the lines indicated
above can be achieved. As you know the resolution that we wished for
concerning Jewish immigration was in fact passed at the congress of
the Arab leaders held in Paris in June of this year, at which Mr.
Hochberg was present. (ibid, p. 215)
But it was hard for them to find people to work with, as there was in fact no political unity among Arabs at that time. In May 1914, the Zionist diplomat Victor Jacobson wrote:
We here are in contact with several gentlemen who call themselves the
chiefs and leaders of the different Arab groups. Each one claims that
it is he who is the real, the only real, the important one... There is
no way of knowing what truth there is in what they say, what is behind
them. They do not have a single organization. (ibid, p. 219)
In conclusion, in 1907 the Great Powers did not consider the Arabs a political force and had no reason to do so, making this quote far more likely to be a forgery or misquote.