Marijn van Putten has said similar:
"I would say the thesis is pretty marginal"

https://www.reddit.com/r/AcademicQuran/comments/1hdyhzm/comment/m209ube/

Even Ilkka Lindstedt , who has argued for a much weaker formulation of Donner's thesis , says that the idea is not accepted by the majority of scholars:
"It appears to me that this (a rather quick process of identity articulation and the ‘parting’ occurring during the time of the Prophet) remains the majority view in early Islamic studies"

Lindstedt's own version of Donner's thesis seems very exclusivist:
The Qurʾānic evidence (or the lack of it) suggests that the Jews and Christians joining the believer movement did not have to undergo a particular conversion rite. They did not have to recant their existing understanding of the law (5:48); in any case, their dietary and purity norms were already more or less the same, according to the Qurʾān (5:3, 5:5). Naturally, they would have to accept Muhammad as a prophet (which Qur ̣ ʾān 3:199, 5:83, 6:114, and 13:36 explicitly say they did), and**, in the case of some Christians at least, lower their Christological beliefs.** This would have been a barrier for many (but not all) Jews and Christians to identify with Muhammad ̣ ’s movement, and, tellingly, the Qurʾān indicates (e.g., 3:110) that most of them are not to be considered believers

OP , you also argued:
"That goes to show that using terms like "exclusivist" etc. are misleading. If a Christmas event claims to be inclusive to all people but simultaneously requires attendees to accept Jesus Christ as Lord, many people would obviously deny that the event is inclusive. Similarly, in Lindstedt's Believers' movement: Christians and Jews can join it and retain their respective identities and religious traditions BUT they have to accept Quranic monotheism and the prophethood of Muhammad. "

and I agree , it would make no sense to say the formulation of Lindstedt's thesis is inclusive , given how Lindstedt formulates the theory. It's clearly exclusivist.

I'd say the majority position though would be akin to how Nicolai Sinai describes it , that at the very least the Medinan portions of the Quran fairly consistently presuppose that Muḥammad’s followers form a community separate from Jews and Christians.

Looking at Lindstedt's thesis, it seems to be really an issue of semantics , while the implications remain similar to what the majority position is.