Do expandable above ground pool liners truly expand

No. Expandable liners are not expandable in the sense that they stretch like a Spandex material does.  Expandable liners have plasticizers which do minimally stretch but there is a specific fixed maximum depth.  They are made of vinyl material that is only made to slightly stretch due to its pliability.  The term expandable liner originates from the concept of a flat bottom above ground pool. Taking a flat bottom pool mindset and expanding it to have a slightly deeper swim area.

Several years ago there were only flat bottom above ground pools.  A fixed depth if you will.  Then a liner was designed that would allow the flat bottom concept to be expanded to include a deeper area for swimming.  Thus the birth of expandable pool liners.  Today, the expandable liner depths now vary from 60" to 72" depending upon the liner manufacture.