Translation:Their defence is the strongest defence that we know of.
Before seeing that, I had a vague notion it meant "resembles"
to resemble = mirip dengan = serupa dengan = menyerupai
So, yes, you're very close to the real meaning of the verb, especially if you look at the base word "rupa" (Sanskrit origin).
"rupa" (noun) = form, shape, appearance.
Here is the KBBI definition of the verb:
1 memberi rupa; membentuk (menjadikan) supaya berupa: ....(example sentence)
2 adalah: ... (example sentence)
3 menjadi: .... (example sentence)
"adalah" is the second definition.
In this sentence, it can also be translated to something like "to give the appearance", "appears to be" (the first definition).
Imagine that the above sentence is a statement from a politician or a military spokesperson.
"Their defense is the strongest...." (second definition, 'adalah')
"Their defense appears to be the strongest..." (first definition, 'memberi rupa')
The first sentence tells me that this person is absolutely sure about the fact that their defense is indeed the strongest.
The second sentence tells me that this person is not so sure at all about it, but only 'knows' by looking at it. This person is only judging by the looks/appearance of it.
'memberi rupa' = 'to give the appearance', 'to have the shape/form'
The fact that something has the same shape/form/appearance does not necessarily mean that it is indeed the same thing.
For me, that's the difference in meaning between "merupakan" and "adalah".
To put it differently, if you translate the English sentence back to Indonesian, and you would use 'adalah' instead of 'merupakan' , then it would sound like that this person is making a factual statement.
For me, the Indonesian sentence is not necessarily a factual statement, but it can also be a judgement call.
Regarding the word "rupa", if you ever go to Borobudur, you can see how this word is used in the original Sanskrit form.
Kamadhatu = the realm of desires.
Rupadhatu = the realm of forms.
Arupadhatu = the realm of formlessness.