Special:CX and all CX code is under beta feature. But if user enabled it in one wiki, choose to translate from that source language to target language, the translation happen in target wiki by design. But if user did not enable CX beta feature, this will result a "Page does not exist" error. That is not desirable and since user already opted for beta feature in source wiki, open the extension for the user in target wiki - But only if all valid URL parameters are present.