On one hand, the Japanese word for "love" can also be used to describe friendships without sounding too emotional from their perspective and this could be a (very rare) case of Nintendo going with a more literal translation instead of taking creative liberties as they often do for Splatoon's english translation.

On the other hand,

Now on the one hand, this can be taken as a simple confession of platonic love. These two have been through a lot together, and so it makes sense that they would care enough for each other that yes, they would call it love, but that does not mean it has to be romantic.

On the other hand, they're actually married and in love shut up