When using gen-class, the generated .class doesn't have the default methods from java interfaces.
I created this repro, you can confirm a exception is throw when trying to call the method from the generated class from gen-class:
clj -T:build jar to generate the classes at target/classes
java -cp clj -Spath
A exception is throw:
Exception in thread "main" java.lang.UnsupportedOperationException: otherthing (clojure-sample.foo/-otherthing not defined?)
at clojure_sample.Foo.otherthing(Unknown Source)
The not ideal workaround is to add the default method in the namespace implementing what is done in the default method inside the java interface.