Strictly speaking it is not plagiarism to mention this minimum viable product in your thesis. That said, however, there are two considerations that require further attention; first, though, a point of clarification concerning "plagiarism":
Per Oxford University Plagiarism Guidelines, "Plagiarism is the presentation of someone else's work or ideas as your own.[...]by incorporating it into your work without full acknowledgement"
There are many forms of plagiarism, some intentional, and some unintentional, but while varied, plagiarism refers to a practice of attribution concerning some intellectual work; plagiarism is not however automatically conferred by a mode of authorship. Simply because a project, as your hackathon represents, involved a group does not preclude your ability to cite that project, properly attributing and crediting the efforts and works of each contributing group member. Such an instance of a group project, one that is directly instigated by a lecture presentation that you made would be permissible to cite as a project with discrete elements and scope. Similarly it would be irregular to cite the entire group in reference to the lecture you made.
Any work that is relevant in scope, or impact, to an intellectual endeavor or to an academic end arguably must be cited, regardless of whether it is convenient; the point is that it could be argued that leaving out the group project, as in not mentioning it at all, would constitute an instance of plagiarism.
Given the scope of your thesis and your stated acknowledgment that the lecture presentation you made and the group project are relevant to that end, you should acknowledge both, including contributors/collaborators.
Per internationally recognized ISO7144 standard, to varying extents, a thesis generally is a theoretical, research-based, or clinical work that is prepared with the aim of demonstrating its creators qualifications.
While your project is important and relates to your theoretical work and research (and as such should be cited with credit to all participants), your project represents an instance of a theory you've developed, relating thus to your presentation in turn; in the same way that it would be difficult to submit a project that was merely practical, lacking research, as the thesis itself, it would follow that the project, which acts a supporting instance of your theory, would not undermine the research you conducted or the theory you promote.
Simply because there are collaborators on a project inspired by your research is insufficient by any count to be considered unfair or unoriginal work to the whole thesis.
Ultimately only you can know the extent to which any of this applies, but keep in mind many academic works are the fruit of collaborations that may or may not be part of a much larger researcher's canon or research field without rendering the whole of his research a group activity.