I agree. This is the first time I’ve ever heard this, and I don’t agree either. What they DO impart however, is the entire signal chain: mic, cab, mic pre, and ( important) conversion. So yes, however someone captured that IR, makes a difference yes. I guarantee you an IR captured with an API preamp and shitty ( or great) converters is going to sound and feel different than an IR captured with a neve style preamp and the same converters ( and same mic and cab obviously). The preamp and the converters mostly definitely make a difference; how big or small it is is up to your ear and what you think. I personally think they matter a lot, and are two factors that don’t get talked about much( if at all) when capturing IR’s.