It strikes me as a misunderstanding on how career evolution works in these companies (and, I believe, most tech companies). A senior eng will focus less on the pure technical aspects (language, framework, etc), but rather will have the soft skills allowing them to carry larger projects/impact.
For instance, with experience, a senior can identify design mistakes early, can weight the value of different solutions based on aspects like time/eng-cost, tech debt, complexity, risk.
In a company of the size of Shopify, shifting away from "Rails ERB templates" to a more modern solution definitely require these skills (and also probably way more than 6 months of tenure). They could have proposed this project, tried to get some buy-in and learn new "frontend skills".