They couldn't maintain a WordPress site without him either.
The full story though is they had their Wordpress site done by a company from Singapore who charged them 1000's of dollars for what seemed to be a $50 template from Themeforest. And the worst part was it was hosted on top of a third grade shared hosting provider, with really low memory and RAM and the site would take about 15-20 seconds just to render the page. It didn't even include a year's worth of support which they would've easily gotten if they directly purchased from themeforest. This company then stopped answering their calls one fine day and abandoned them after taking their money.
The site itself was fine, until one day it broke and was completely inaccessible.
So, they were stuck in a limbo and the CEO happens to be my good friend, so while working on another project for him, he asked me if I could help. So, that's how I got incharge of maintaining it for them.
Sorry if my original post wasn't clear. I'm running with Jekyll + Jekyll admin for one of them and I use Netlify Admin for another. So, they can still use a WYSIWYG interface to edit their content without having to deal with code.
You usually can't just replace Wordpress with Jekyll and call it a day since non-technical people need to maintain the site. Though it's super nice when you can.