I've found that agencies are behind the times when it comes to design tools. I once exported a screen of a mobile UI from Sketch as a PDF, only to be told "When I opened it in Photoshop, it was one flat layer. Can you send over your original Photoshop file?" I told them that the original was made in Sketch, and that was the last I heard from them.

To be fair, Sketch is a Mac only program, so the team may have just been unable to use it, and preferred to just move on rather than purchasing an entire new computer (and Sketch). It would perhaps been more helpful to have responded by offering to provide your Sketch assets/layers via Zeplin, which is free and web based.

Sketch files also open in Figma which is free, browser based, and OS agnostic.