Matrix exclude not working for multiple env strings

I have the below travis.yml config. I am trying to exclude a build when compiler is clang and ENABLE_COVERAGE is not true. I have been able to exclude to work when the env line only contains a single variable, but not when it has multiple values. Any help? Is this supported?

sudo: required
language: c
  - docker

  - gcc
  - clang

  - ENABLE_COVERAGE=true DOCKER_TAG=ubuntu-16.04
  - ENABLE_COVERAGE=false DOCKER_TAG=ubuntu-18.04

# only one build should report coverage stats
  - env: ENABLE_COVERAGE=true
    compiler: clang

Link to Build:

Please read

When excluding jobs with env values, the value must match exactly .

You have ENABLE_COVERAGE=true DOCKER_TAG=ubuntu-16.04, so that exact string must appear in matrix.exclude as well.

That seems like quite a limitation. I guess perhaps the better approach would be to include it rather than filter it out.