following

I think you are right. Any prototyping tool has certain affordances that make it more likely to come up with a certain set of idea's and limits the imagination to this set to some extent. These are two sides of a coin. Some prototyping tools are more flexible (such as sketching) than others (such as wireframing toolkits). The more flexible a system, the less fixation.

Arduino makes it easier to come up with ideas for sensor - actuator systems, that would be much harder to imagine and articulate without this prototyping material - but as a consequence it also focusses our attention to these types of solutions. I do think it is a fairly flexible prototyping tool and it is hardly the only one that designers use. So in the total repertoire of design prototyping it is a valuable addition.

Design fixation is intrinsic to the designer's personality, capabilities, propensities, etc. and is independent of the immediate and present external world. All tools at some point become limited (and limiting) and they are eventually extended by the demands of those designers who are not anchored by them in their design creativity. In contrast to the "tool extenders" others may be trapped by comfort and success in using a specific tool, and often channel their creative trajectory by what the tool affords them. For example, following Koen Van Turnhout's example, most instructors in product design challenge students to keep sketching with the assumption that sketching is less limiting then CAD, at least in the morphological aspects. It is not always the case, but mostly true, at least with students at earlier stages. Same with Arduino, Fritzing may be more liberating, but then at some point someone will run out of canvas there and will need to extend to another tool (e.g. VHDL equivalent).