The study in question is actually Hoyland's article "The Earliest Christian Writings on Muhammad. An Appraisal" (2000). Confusingly, this article "The Earliest Christian Writings" can be found on Hoyland's Research Gate page under the title Writing the Biography, but the actual article "Writing the Biography" can be found elsewhere.

At any rate, the quoted line comes from p. 292 of Hoyland's "Earliest Christian Writings" article. Hoyland's footnote to this sentence leads us to Camilla Adang's Muslim Writers on Judaism and the Hebrew Bible (1996), pp. 141-43. These pages are part of a longer chapter on The Proofs of the Prophet. Adang begins with how the Quran itself makes mention of biblical texts (here's a modern compilation), which she interprets as evidence that from the very beginnings of the Islamic community, Muslims were challenged to present themselves as continuators of biblical traditions, especially the traditions from the Hebrew Bible or Old Testament. That's a pretty straightforward claim, since early Muslims presented themselves as a sort of reform movement within the Abrahamic faith, so of course they had to reckon with the ways in which Jews and Christians curated and interpreted their own scriptural canons.

The particular points of dispute, however, were raised by early Christian and Jewish critics of Islam by at least the 700s CE. According to Adang, these non-Muslim scholars had two main complaints: (1) Muhammad was not anticipated by any of the earlier prophets, and (2) Muhammad's career was not accompanied by any miracles showing a divine endorsement of his work. Muslim scholars responded by presenting their own interpretations of biblical texts.

For the first point, Muslim theologians simply argued that Old Testament texts in fact anticipated a prophet like Muhammad and not a Christ like Jesus. (Jewish scholars, presumably, would have supported at least part of this argument.) For example, in an early text prepared by the scholar Ibn al-Layth and sent by Harun al-Rashid to the Byzantine emperor around the year 796 CE, biblical citations included Isa 21:6-9, Ps 9:20, Hab 3:3-6, Ps 149, Isa 42:10-12, Isa 42:1-4, Ps 45:2-5, Deut 33:2, and Deut 18:18 (taken from Adang's p. 144 footnote 24).

Note that these are all texts from the Hebrew Bible or Old Testament, but many of them were also ones that Christians drew on as purported prophecies of Jesus. Without looking up particular passages, this is already clear from Ibn al-Layth's preference for advancing arguments based on Isaiah and the Psalms, which are key texts referenced time and again throughout the New Testament. Interestingly, not only is this the earliest known list of testimonia or Biblical proofs adduced to demonstrate Muhammad's authenticity, but Ibn al-Layth also referenced the New Testament at various points to chastise the Byzantine emperor for not being a more peaceable Christian. He clearly knew his way around the Christian and Hebrew bibles!

The second point—whether Muhammad worked miracles or was endorsed by divine signs—seems to have been more important to Jewish scholars in particular. (This perhaps suggests that some Jewish scholars thought Muslim arguments about Old Testament prophets at least might be sound!) This problem could be dealt with in a number of basic ways. Muslims could reject the need for miracles, they could claim that Muhammad had worked miracles on Jewish or Christian terms, or they could reinterpret Biblical texts so the criteria used to define a miracle more closely aligned with things that were commonly believed to have happened. They seem to have been experimenting with these diverse lines of thought by at least the late 800s CE, if not earlier. Perhaps one of the most enduring results of this debate is the common belief among many Muslims that the Quran itself should be considered a miracle, equivalent to or even superseding the miracles worked by Jesus and Moses.

In a way, late-antique Christians made things easy for early Muslims. Early Christian thinkers had already identified Old Testament passages that seemed to anticipate the coming of Christ, and early Muslim thinkers simply needed to show how Christians had misunderstood these things and Muhammad was in fact a better fit. Whether or not we consider them successful just might boil down to a matter of faith.

*Edit. Added links to biblical passages.