Yes. you are right.
TGZ is a steamable format, but Bandizip does not support it.
In most cases, the extraction sequence is as follows.
1) An user opens a TGZ file.
2) Bandizip extracts the file to memory and reads the content of the file. - 1
3) User select files to extract
4) Extraction. - 2
If you want to extract a TGZ file and you don't need to check the list of the file, the sequence could be done at once like this.
1) Extract the file to memory -> Read Contents -> Write it to HDD
But, Bandizip does not support it (because it's not that easy.)