Hello! This is absolutely a fair question. While it is true that case interviews are conducted by senior consultants (project leader/engagement manager level and above) with the rare exception, junior consultants at most firms are also able to get heavily involved in the recruiting process, which gives them experience in preparing candidates and a deep understanding of the interview specifics.
From my personal experience at McKinsey's office in San Francisco, junior consultants would often get involved in running mock interviews with candidates which the firm offered as part of the recruiting process, facilitating case prep workshops, and becoming a point person for recruiting at their alma mater university. There was even a special "West Coast Ambassador" role for junior consultants to work in recruiting as a full-time project for several months, whereby they planned and ran many case workshops and mock interviews (and occasionally a few first round interviews). When I was a candidate interviewing with McKinsey, I was also assigned a McKinsey mentor after I passed my first round interviews, who was a junior consultant specifically tasked with running a few cases with me and answering any questions I had about McKinsey's interview process. Other consulting firms also have similar programs.
So, while junior consultants will rarely have had experience of running the official case interviews, they will often have had opportunities to get involved with the recruiting process, becoming very familiar with their firm's interviewing methods as a result. In addition, junior consultants bring the added bonus of having gone through the interview process more recently, so their preparation and what they did to be successful is likely to be more fresh in their minds.
However, how this information should pertain to your decision to work with a junior consultant as a coach is an entirely personal matter. In my experience as a candidate, some of the most helpful coaches I worked with were ex-junior consultants doing their MBAs. My approach was to place less emphasis on the seniority of the coach, and more emphasis on trying to get more case prep in with a coach who could offer me any useful advice, so I was more than happy to work with junior consultant coaches, particularly as they frequently charged less per session. I believe I benefitted from this approach - but your cost-benefit analysis might be different, so it is totally up to you!
Please feel free to DM me if you'd like more details on any of this, and best of luck in your case prep and in picking a good coach!