I think there is an anomaly in the rules here. An STI can teach in a simulator only, but I was told some time ago that a prerequisite to getting an STI was to have held an appropriate instructional qualification in the preceding three years. So you could have had an FI 4 years previously which you would have to renew in order to then gain an STI. If there was a medical issue then you (having been a ME instructor four years before) could, no matter what you did, ever get an STI.
Yet a 10 year lapsed STI could do appropriate refresher training and then regain their STI. Doesn’t make sense to me.