The Long Lance was designed to threaten any capital ship

What would happen if a Lexington met the wrong end of a Long Lance torpedo?

It would face the same grave danger as other capital ships because the Long Lance was built to inflict decisive damage from beyond secondary-gun range.

Any ship struck by a Type 93 Long Lance faced a severe danger. Lexington's light protection would not make such a hit a special or surprising weakness. The weapon's operational purpose was to let Japanese light forces kill or cripple battleships from outside the victim's secondary-battery range. The exchange therefore reframes the torpedo as a fleet-wide tactical threat rather than a Lexington-specific design test.