Syntactic role of "我" in "我真是这个意思"
I believe the grammatical structure of 我真是这个意思 is:

Subj: 我 - I
Adverb: 真 - really
Verb: 是 - to be
Object (Pronoun + Measure Word + Noun): 这个意思 - this idea

Which literally means I really am this idea. However, this use of the verb to be indeed sounds strange: the person I is not an idea!? But, we can understand it considering the 'illogical' sentences of the copula verb 是, in which 是 has a metaphorical meaning understood from the context. For more details see the answer I referenced, but we can understand the main idea with an example:

我是炒饭.

Which does not mean I am a fried rice (literal), but implies (e.g. in a restaurant) I am the one having the fried rice (metaphorical). The same occurs in the common expression:

这是什么意思?

Which does not mean This is what meaning? (literal), but implies What does this mean? (metaphorical). So coming back to our example:

我真是这个意思.

Which does not mean I really am this idea (literal), but implies I really mean this idea (metaphorical).