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The Gospel of Marcion is said to be evidence that the Gospel of Luke was in fact written by Luke:
Tradition unanimously affirms this author to be Luke. This is attested by the early heretic Marcion (who died c. AD 160; Luke was the only Gospel in his canon), ... — Luke, Tyndale New Testament Commentaries
In what way (if any) does the Gospel of Marcion attest to Luke's authorship of the Gospel of Luke, given that Marcion did not attribute authorship to Luke?
This was not, perhaps, Leon Morris's finest moment (quote is on p. 17, originally published 1974), although he certainly wasn't alone in assuming this datum. Neither Howard Marshall, nor John Nolland make mention of Marcion in their circumspect discussions of the attribution of authorship of the third (canonical) gospel -- simply to cite two subsequent "heavy-weight" commentators handling the same topic.
Tradition
The tradition that Marcion's "gospel" was a form of Luke goes back at least to Tertullian (c. 160 – c. 225 AD), who wrote in his Adversus Marcionem (in Bk IV.2):
Now, of the authors whom we possess, Marcion seems to have singled out Luke for his mutilating process. ...
Tertullian goes on in this treatise to assume this identification, and so proceeds to castigate Marcion for neglecting the other gospels in his mutilation of Luke (Bk. IV.5):
Well, then, Marcion ought to be called to a strict account concerning these [other Gospels] also, for having omitted them, and insisted in preference on Luke; as if they, too, had not had free course in the churches, as well as Luke's Gospel, from the beginning.
Thus the tradition of identifying Marcion's "gospel" with Luke has a long pedigree, and was to a large part accepted.
Current assessment
However, this assumption was already being probed in, e.g., John Knox's study, "On the Vocabulary of Marcion's Gospel", Journal of Biblical Literature, 58 (1939): 193-201. He concludes (p. 200):
The Gospel of Marcion, as we are able to reconstruct it, does not show enough of the literary character we have learned to associate with Luke-Acts to establish its homogeneity with that work.
A key phrase there is "as we are able to reconstruct it". Here discussion rumbles on. See, e.g.:
Conclusion
The strong tendency in current scholarship is to resist identifying the gospels of Luke and Marcion. So if the question is: "Does the Gospel of Marcion attest to Luke's authorship of the Gospel of Luke?", the short answer is "No, given the present understanding of what actually constituted Marcion's gospel". And the even shorter answer, then, is: "No".[full-stop]
Note: see also @fdb's comment to the Q&A noted under "Linked" in the sidebar. However, the fact that the fragments cited by Tertullian do not contain an attribution to Luke didn't stop Tertullian from making that identification: he made it quite insistently.
