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
services:
  - docker

compiler:
  - gcc
  - clang

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

# only one build should report coverage stats
matrix:
  exclude:
  - env: ENABLE_COVERAGE=true
    compiler: clang
<snip>

Link to Build:
https://travis-ci.org/tpm2-software/tpm2-tools/builds/578516443?utm_source=github_status&utm_medium=notification

Please read https://docs.travis-ci.com/user/customizing-the-build/#excluding-jobs-with-env-value:

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.

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