Is this formula describing the loss of unfixed genes from a population accurate?
The expression is based on a real one, but missing a minus sign (EDIT: not anymore! See updated question), and the explanation is garbled. For the actual equation, see "eigenvalue effective population size" in, e.g., WJ Ewens, Mathematical Population Genetics. There should be a negative sign in the exponent: $L_T = L_0 e^{-T/2N}$. Note that you need this minus sign so that "genes" (actually heterozygosity) are lost with time rather than growing exponentially in number!

As for the errors in the explanation: $T$ should be measured in generations rather than years, and $N$ should be the number of individuals (or rather the "eigenvalue effective number") instead of the number of generations.