"Full employment" absolutely builds in the reserve army of labor power into the definition.
https://fred.stlouisfed.org/series/LNS11300001
Was 86% or above before 1950, never above 70% over the past 5 years. The trend has been downward the whole time.
From 2000 to 2020, the labor participation for men from 20 to 44 has fallen. Labor participation for men 45 to 54 varies from flat to down. The labor participation rate for men 55 and older has risen. Working age men are less employed than they were, retirement age men are more employed than they were.
https://www.bls.gov/emp/tables/civilian-labor-force-particip...
A lot of people that are not technically unemployed are also not working fall into a few groups: those that are dependent on others to work; those that are too young or too old to work; and those that have simply given up on the whole notion of working because they are no longer able to physically or mentally (or both).
And there is of course a big problem with poverty even among those that work. So unemployment figures look great. Poverty figures, not so much. IMHO basic income would be a fair and simple way to ensure we get rid of poverty that should not break capitalism.