Yes it is.

- Flight itself goes under airport control at t - 48

- When it comes to seats, assuming you are talking Y, there are three categories of seats in the cabin: (1) seats reserved for elite passengers, (2) seats available for pre-allocation by anyone, (3) seats not available for pre-allocation until OLCI opens at t-30 (or earlier for people who check in with connections). The person you spoke to was correct in saying that the free seats that would be available for non-elite passengers are under airport control and are not available for pre-allocation. In other words, the flight is not overbooked at all (current availability in Y is Y9 B9 M9 U9 K9 H9 L9 Q9 T0 E0 N0 R0 V0 X0 G0, so they must have sold quite a few as all the cheapest buckets are 0'ed out but clearly plenty of availability in medium and high Y fares) but all the seats available for pre-allocation by all passengers have already been assigned. The only seats available for pre-allocation left are the priority (elite and elite+) seats and there is 0 chance that they will allow non-elite to pre-assign those. However, any such seat not pre-allocated as well as all the seats that were never available for pre-allocation will become available when you family OLCI 30 hours before their IAH flight and/or when they go to IAH to check in.