This is correct. In the Gen. 1 and Gen. 2 product the extra DSP resourcess for the L2 were hardware on a daughter board that wasn't installed on the L5. I believe the Gen. 3 L5 actually has the 60 DSP resources populated in hardware but they're crippled in software to only allow 30. I haven't opened one up to compare but that's what our Adtran rep told us. This shouldn't be an issue in real life unless you have more than one PRI and all of the FXS ports are in use on a very busy box, or you're doing a lot of T38.
FYI, if you need exactly two FXOs there's a product that is kind of a sleeper, the Netvanta 6240. It is virtually identical to the TA900 Gen. 2 series in all respects but comes with two FXOs in all models. Other than the extra FXO on the RJ-11 (now an RJ-14) and the case color you literally can't tell them apart.
There's also the Netvanta 6250 which is basically a TA900 Gen. 3 with four 100-mb Ethernet ports. Also comes with SFP option on the gig Ethernet port. Only one FXO on those, however.