Some random thoughts:

The two fire team Army squad can't deploy one up, two back as is; however, the lead squad in a platoon movement to contact usually has a machine gun team directly attached, making it a defacto three team squad.

A two team squad alone will often still be suppressing with about 2/3, even though it's not configured in thirds. Often, out of an entire squad, only two or three men are moving at once. One fire team suppressing and one assaulting or bounding forward, of course, but not everyone in the assault team is moving at once. Part of the assault team is suppressing too.

I can see how the 13 man Marine squad is advantageous; however, is the advantage so great that it's worth the doctrinal change? The two team squad Army squad has been doctrine since about 1957. How long does it usually take for new doctrinal thinking to permeate a service culture at the tactical level?

All things considered, maybe the best thing for the Army to do would be to keep the two team squad but go back to larger fire teams.

What am I not seeing clearly?