I've set up a Developer Key at my Account and configured it to allow Link Selection placements of the LtiDeepLinkingRequest message type. As well, I've set up a pseudo-tool at IMS's LTI Reference Implementation Test Tool to use its provided DeepLinking Launch URLs with the configured Developer Key, so that I can select the content to Deep Link from a UI.
I tried to add an External Tool-based Module Item to a Course and verything went well at first, but it seems that I can not select non-LTI Link contents from the Tool Provider (in this case, the pseudo-tool from IMS-RI): if I select only another kind of content, such as an HTML Link, whenever I send the selection back to the Module Item creation modal I'm getting an error pop-up with the message "Selected content is not an LTI Deep Link". I've also inspected the sent DeepLinking Request claims, and found out that the https://purl.imsglobal.org/spec/lti-dl/claim/deep_linking_settings claim has its accept_types sub-claim set to an array of only "ltiResourceLink", which I could not configure differently.
Maybe I'm not understanding this doc's Limitations section either, but my question is: Is it possible to select contents other than LTI Links for Module Items through a DeepLinking flow?