Longterm catabolic steroids - like triamcinolone - cause muscle weakness, but the effect of a one off injection is what is of interest here - and I suspect traditional doctors basically don't know, and of course they don't know what they don't know. Steroids can cause euphoria which could help performance, and they also reduce inflammation from almost any cause, and cyclists will have muscle inflammation, and associated fluid retention, from all the training - this is my understanding of how steroids result in weight loss, and a consequent improvement in power to weight ratio.
The doctors working with the cycling teams will almost certainly know how effective steroid injections are, but they are not going to tell us.