On the validity of the study:
It actually is just one of the latest in a host of scholarly research papers (by him and others) that basically say the same thing.
See the work of Megan Spencer, Daniel J. Milton, Cameron Harris, Kenneth Rogerson Justin Conrad, William Berry and Matt Golder, to name a few, in peer reviewed publications like The Journal of Politics, The Journal of Human Rights, International Interactions, the Journal of Information Technology and Politics, Studies in Conflict & Terrorism, etc.
Essentially, keeping the refugees close to Syria is akin to storing your firewood & propane next to your bonfire.
The International Security and Defense Policy Center of the RAND National Defense Research Institute, a federally funded research and development center sponsored by the Office of the Secretary of Defense, the Joint Staff, the Unified Combatant Commands, the Navy, the Marine Corps, the defense agencies, and the defense Intelligence Community recently presented similar data to the Office of the Secretary of Defense in a report by William Young, David Stebbins, Bryan A. Frederick and Omar Al-Shahery.
The paper presents a number of factors increasing the likelihood of refugee radicalization and the spread of open warfare to neighbors countries:
1) several of those countries sponsor terrorist groups or are otherwise engaged in efforts to destabilize their neighbors.
2) the region is the base of operations of numerous terrorist groups who can operate with impunity. With the region's porous borders, jihadists may freely travel to and from battlefields and refugee camps.
3) the resources allocated to help refugees in surrounding countries are already near breaking points, and the demands of supporting those camps are seen as being in conflict with meeting daily needs of their own citizens
Example: Jordan has 6.7M citizens, and they have accepted 670,000 refugees- IOW, @ 1/10th of their population. They don't have adequate resources to clothe, feed, house and- key to this discussion- check out & monitor all of those people. Hizbollah, Iran’s Quds Force and agents of Damascus are known to have carried out attacks in Jordan. They have also tried to radicalize the refugees. With easy access to the refugees, terrorists have large and receptive recruitment pool.
Lebanon, with a population of 4.5M has taken in @1M refugees, face the same issues of jihadist operations, have even fewer resources...and is more politically unstable.
The RAND report thus predicts a high probability of Jordan & Lebanon becoming new fronts in open hostilities if the Syrian crisis isn't resolved soon. At no point in the study do they take seriously the notion that giving "more aid over there" will be a major factor to stem the tide of radicalization. Instead, they conclude:
Policy measures that are focused solely on the effects of the spill- over (such as helping Turkey, Lebanon, and Jordan deal with the flow of refugees within their borders) are unlikely to be sufficient—like a doctor treating only the visible symptoms of an infectious disease: The patient and others standing nearby will continue to be at risk.
IOW, that study's assertion is pretty non-controversial stuff; it is the mainstream position in the community of experts and administrators dealing with refugees.