If you want to transfer a site, yes. You could have worked on your clients’ webflow account but this is not a very good practice. We’ve asked for an in-between, better solution to collaborate with clients who already have a site or webflow account. I’m sure a solutio will come one day.
Be aware that transferring a site is… transfering the site, not transfering a copy of the site. Once you click OK, you don’t have the site anymore. So you may want to duplicate the site prior to transfer it, so you got to keep a copy for reference and whatever future needs.