This is classical apologist trick where one takes ancient text/language and re-interpret it to mean what ever you like. For instance corpus.quran.com provides competing translations for this specific verse:
Sahih International: And We placed within the earth firmly set mountains, lest it should shift with them, and We made therein [mountain] passes [as] roads that they might be guided.
Pickthall: And We have placed in the earth firm hills lest it quake with them, and We have placed therein ravines as roads that haply they may find their way.
Yusuf Ali: And We have set on the earth mountains standing firm, lest it should shake with them, and We have made therein broad highways (between mountains) for them to pass through: that they may receive Guidance.
Shakir: And We have made great mountains in the earth lest it might be convulsed with them, and We have made in it wide ways that they may follow a right direction.
Muhammad Sarwar: We placed firm mountains on earth lest it would shake them away. We made wide roads for them so that they might have the right guidance.
Mohsin Khan: And We have placed on the earth firm mountains, lest it should shake with them, and We placed therein broad highways for them to pass through, that they may be guided.
Arberry: And We set in the earth firm mountains lest it should shake with them, and We set in it ravines to serve as ways, that haply so they may be guided;
Even if we were to grant that interpretation of 'We placed firmly embedded mountains on the earth, so it would not move under them' was more correct, it is not consistent with plate tectonics. Mountains are formed when plates press against each other. wikipedia/Mountain_formation