This paper addresses the critical need for inclusive, human-centered, and neurodiverse research paradigms within the field of immersive extended reality (XR) technology interventions for transition-aged autistic adults. Specifically, we explored the use of extended reality XR as an intervention tool for autistic adults, employing a community-based participatory research framework. Project PHoENIX, an XR training experience, was assessed for usability, social validity, and alignment with the needs, preferences, and values of autistic stakeholders. Data collection involved the Computer System Usability Questionnaire, an open-ended questionnaire, and a social validity instrument, complemented by videos of focus groups. A combination of qualitative and quantitative methods was used for data analysis, including content analysis of open-ended responses and spreadsheet calculations for CSUQ and social validity questionnaire data. The results are structured around three research questions, addressing usability, social validity, and stakeholder needs in XR design. The study highlights the need for participatory, inclusive, and flexible design approaches in developing XR tools for the autistic community, acknowledging the diverse experiences and requirements within this population.