Why old submarines were not a quick answer to force shortages

Could old submarines be recommissioned quickly if submarine numbers had to grow?

Probably not. Some T-class boats are the only possible exception mentioned, and even that is doubted.

Retired submarines were unlikely to provide a short-term increase in boat numbers. Some T-class boats are the only possible exception mentioned, and even that possibility is treated with skepticism. No specific hulls or reactivation plan are offered. The passage therefore supports only a cautious conclusion.