I know that. I'm talking about further down the line for 5G. What I'm saying is that with the addition of Sprint's PCS, they can expand LTE on PCS and shift HSPA from AWS to the remaining 5MHz in PCS-B so that they can have a 25MHz n66 carrier down the line and a 20MHz n2 carrier since n2 is not currently registered for greater than 20MHz carriers.
The other solution would be to simply get rid of HSPA altogether and have two 25MHz n66 and n25 carriers since n25 allows for 25MHz carriers. Then T-Mobile could retain the 5MHz PCS-G for legacy LTE service.
They poached it from the lower 5MHz of Sprint's PCS B-block. I'm assuming the plan is to eventually expand PCS to 20MHz, leaving two 5MHz blocks (one in B-block and one in G-block) for legacy HSPA and Sprint LTE.