"No stronger" means all the others are either weaker or equally strong. The latter must have been suggested by Obama with respect to other NATO countries, to stick to the rules of diplomacy. Probably he wanted to say "We neither have a better friend nor a stauncher ally". There can be a strong friendship to a weak person.

When politicians speak about friendship between countries, they actually mean particular governments in a particular moment.

Diplomatic bla-bla has nothing to do with ordinary people and should not be allowed to affect their emotions.