If one were to submerge lettuce in a pot of boiling water, cover the pot with a lid, let the lettuce boil for a minute and then discard the water; would doing so remove all the bugs?
The method you describe is not one of the recommended ones for checking lettuce. The critical issue is whether you would be killing bugs who would stay nested in the folds of the lettuce leaves.
See the OU guidelines on checking lettuce for instance
The insects most commonly found in open leaf lettuce are small green aphids or thrips. The leaves of the vegetable often camouflage these insects. The open structure of these vegetables allows insects to penetrate the entire head. Often, insects may be found between the innermost layers of leaves of an infested head. Therefore, each leaf must be washed and checked individually. Many of these varieties feature curly leaves with many folds in which the insects tend to hide. We therefore recommend that they be washed and checked with extreme caution.