While the basic syntax is relatively easy, knowledge of APIs (whether standard or third-party libraries), design patterns and conventions takes time (and practice) to master regardless of skill level.
Being an expert at a single general-purpose language will often make you more productive as you can focus on the business problem at hand rather than spreading yourself thin across different languages where you'll perform worse until you become an expert at them (which is unlikely for anything more than a handful of languages and even that requires working with them regularly).
Deep expertise isn't as useful day to day as we often portray. With a little elbow grease one can get to a place of being more than baseline productive in relatively short order. Depending on the language I expect a reasonably good dev to be able to be "fluent" in the range of a few weeks to a few months. And it's not like they'd be useless the entire time before that happens.
To me, that's a reasonable investment to make.
Edit: This does require a certain persona in the space of "reasonably good dev". People who have spent their entire life focusing on a single language or paradigm are a lot less likely to be able to shift gears. People who have broad exposure to different concepts are more able to say "How do I do X in Y?" and stop fighting their new language.
I've seen huge differences in productivity between people who've been working with a given stack with for years vs months.
The quality just isn't even close. Recently I outsourced a project to two groups of people. A jr dev who had 2 years of experience but all of that experience was with the tech stack, and 2 sr devs (10 yrs experience) who had < 6 months experience with the tech stack. And the jr dev (who was an inferior dev in general) blew them out of the water because of the tech stack experience.
And honestly in terms of raw talent I think the sr devs were better, but it just didn't make up for the lack of tech stack experience.
What sort of time are we talking about on these projects? If you're looking at something that has to be done in a few weeks, sure. In a normal role where I expect the person to stick around for a couple of years I'll take the person with better overall skills every day.
In your example it the answer may be “none”, but I’d be curious of the amount of tech debt that may have been introduced by the jr vs the sr’s; whether or not sr’s had better intuition.
Being an expert at a single general-purpose language will often make you more productive as you can focus on the business problem at hand rather than spreading yourself thin across different languages where you'll perform worse until you become an expert at them (which is unlikely for anything more than a handful of languages and even that requires working with them regularly).