Article 310 allow signing state to make a declarations, but clearly say it may not "purport to exclude or to modify the legal effect of the provisions of this Convention". It does not simply say, because China make a declaration, it may choose not to be bound by any of the UNCLOS article.
"Article 310. Declarations and statements "Article 309 does not preclude a State, when signing, ratifying or acceding to this Convention, from making declarations or statements, however phrased or named, with a view, inter alia, to the harmonization of its laws and regulations with the provisions of this Convention, provided that such declarations or statements do not purport to exclude or to modify the legal effect of the provisions of this Convention in their application to that State."
Article 287 and 298 specified that if dispute occured, state can choose which forum the dispute be settled or exclude the compulsory binding procedure. It does not mean China can arbitrarily draw boundary line that does not conform UNCLOS.
"Article 287, paragraph 1, provides that States and entities, when signing, ratifying or acceding to the Convention, or at any time thereafter,
may make declarations specifying the forums for the settlement of disputes which they accept."
"In addition, article 298, paragraph 1, allows States and entities to declare that they exclude the application of the compulsory binding procedures for the
settlement of disputes under the Convention in respect of certain specified categories kinds of disputes"
None of article 310, 287, 298 say China may not legally bound by UNCLOS.