Thanks for this.
I also emailed Alamy and recieved the following response which also confirms what you have just said:
"If you have captions and tags embedded in the IPTC of your images, these will be imported as captions and tags when you upload them to Alamy and they can be used in our search engine to locate your images. You can remove any tags you don’t want on the images after they have passed QC in Alamy Image Manager. When customers download images, they do not get the metadata embedded that you have added. This is stripped and we add our own including the Alamy credit line."