1)
The verse does not say what they called the Mizbe'ach!
1.
Rashi: This is an abbreviated verse; they called it 'Ed'.
2)
How is the Mizbe'ach a witness that Hashem is Elokim?
1.
Malbim: They will not switch the place of His Mizbe'ach elsewhere, or [offer to] another god.
