The default disambiguationspage message parser in page.isDisambig does the following
message = self.site.mediawiki_message('disambiguationspage').split(':', 1)[1]
That works well when the message content is 'Template:disambig' , which is very prevalent in the i18n messages, however it will fail for most of the following
$ git show d4018551b09b88b3b45bed3eb6b5d2828ade0bf9 | grep disambiguationspage | egrep -v '(Template:|disambiguations-text|This message)' -'disambiguationspage' => 'সাঁচ:দ্ব্যৰ্থতা-দূৰীকৰণ', -'disambiguationspage' => '{{ns:template}}:Milyoltaca', -'disambiguationspage' => '{{ns:template}}:Čvor', -'disambiguationspage' => 'Кеп:цхьатера маьӀна доцуш', -'disambiguationspage' => '{{ns:template}}:disambig', -'disambiguationspage' => 'Project:Σύνδεσμοι_προς_τις_σελίδες_αποσαφήνισης', -'disambiguationspage' => '{{ns:project}}:Naisc_go_leathanaigh_idirdhealaithe', -'disambiguationspage' => '{{ns:template}}:disambig', -'disambiguationspage' => 'سانچہ: ڈسایمبگ', -'disambiguationspage' => '{{ns:template}}:ايرىق', -'disambiguationspage' => '{{ns:template}}:Айрық', -'disambiguationspage' => '{{ns:template}}:Aýrıq', -'disambiguationspage' => 'नमूना: निवारण', -'disambiguationspage' => 'سانچہ: ڈسایمبگ', -'disambiguationspage' => 'sujhi', -'disambiguationspage' => '{{ns:template}}:disambig', -'disambiguationspage' => 'ದ್ವಂದ್ವ ನಿವಾರಣೆ', -'disambiguationspage' => 'Майык: уш-бажы билдинмес', -'disambiguationspage' => 'سانچہ:ضدابہام', -'disambiguationspage' => '{{ns:template}}:Disambig',
Most of those problems would be fixed by T103080: Link should recognise {{ns:Project}} in text if Link was used.