Hi Remi.
in reactive you need to load the Javascript separately: use system client action RequireScript to load the script.
so in summary, you would have an injectHTML block with:
"<a class='twitter-timeline' href='https://twitter.com/TwitterDev?ref_src=twsrc%5Etfw'>Tweets by TwitterDev</a>"
and the RequireScript action with:
"https://platform.twitter.com/widgets.js"