I’ve had my stuff featured on Instagram a number of times and some of those times I did need to send hi-res pics via email, or upload a video for them. However, in the cases of the video stuff, I sent them dropbox links, I did not upload to their site. I suppose the main concern is that they might be able to hack your machine during the so-called process of uploading somehow, which I know nothing about, but would not doubt is possible.
Are they asking for payment?
I’m curious what others might say on this, but I think as long as there’s no exchange of critical information like payment, bank accounts, or ID stuff then uploading to their site might be the main risk. Tell them you’ll Dropbox or WeTransfer, Hightail, etc. I’m not aware of any potentially legitimate reason why they would say no to that. Especially if you have dropbox or similar, you can simply place the link in the DM chat.