The promised seed made to Ishmael was established when he was born.
Gen 21 only deal that Isaac as the 2nd born also included in the covenant ..yes for prophet Isaac it is the covenant for the jews but through Ishmael the covenant should be for muslims and universally binding
When we look at the Gen 21-12-134 it is clear that Prophet Muhammad is the promised seed of Ishmael.
We should treat the passage 12-13 as a whole.
כִּי בְיִצְחָק יִקָּרֵא לְךָ זָרַע וְגַם אֶת בֶּן הָאָמָה לְגוֹי אֲשִׂימֶנּוּ כִּי זַרְעֲךָ הוּא
kî bĕyiṣḥāq yiqqārēʾ lĕkā zāraʿwĕgam ʾet ben hāʾāmâ lĕgôy ʾăśîmennû kî zarʿăkā hûʾ
Here the Hebrew word “et” אֶת־ can mean a definite direct object or “with”.
So the correct translation should be
“For in Isaac a seed shall be called to thee, and also with the son of the handmaid for a (great) nation I will make him for your seed is he“.
Clearly there are two prophecised seeds here..one for Isaac and the latter for Ishmael.
While the first seed through Isaac (v12) can refer to prophet Jesus (as he said the was promised before Abraham Jh 8:58),
….. the pronoun used in 13 הֽוּא׃ /hûʾ he inevitably can only refer to Prophet Muhammad as he is the seed of Ishmael the son of Hagar as the text indicates.