ThinOS does not have the Citrix Receiver installed. What it has is feature compatibility, and these features may be upgraded with every firmware release if necessary. Keep in mind that some of the features of the Citrix Receiver are either Windows or Linux specific and may not be available on any other OS. If you need to upgrade the Citrix Receiver features, you need to upgrade the firmware. The current version of ThinOS (8.4) or ThinOS Lite (2.4) supports XenDesktop 5.6 through 7.13 and XenApp 6.5 through 7.13.
Hope this information helps.