What is it called when people, e.g. computer programmers, pronounce, say, 65,536 as 'sixty-five, five, thirty-six' i.e. omitting 'thousands' etc?

It comes down to individual lethargy and scheduling perception. It would make sense to say the individual digits rather than the larger alternative form even though they are both read in the same direction. It would save a few seconds to exclude the suffix eg. thousands or thousandth. It's quite a mouthful when you get up to the millions and billions if you are behind schedule.

As a tradesman who is upgrading I can certainly say this saves a lot of time at the end of a shift when having to fabricate a few hundred pieces daily. If you are reading out a measurement for an apprentice to "fab" while you, the journeyman, continues the installation or construction process. Something as simple as 23" ⅛" can be read as "twenty-three and an eighth", "twenty-three and one-eighth" or "Twenty-three inches and ⅛ of an inch" where the former phrases are a few words shorter for the same amount of information.

It comes down to familiarity with the job at some point, therefore those who are also in a similar role will understand even without those details. You would save quite a bit of time.