Afaik many app of this kind do not support cross platform at the start. Because what may seem good business sense is not supportable due to resources.
Pc users and Mac users will have many unique issues to each user group which means you would not only need experts for both Mac and Pc architecture but also two sets of support resources. All of which double overheard.
Also I think the background of many of the developers at tumult used to work for Apple. Which seems why it is written for Macs first.
I think at some point they may give you what you want when they reach a stage that they can support it. which would be good.
All of this is opinion based by the way and in no way trying to say any of this is fact where Tumult is concerned or that this is their view and as such may be completely wrong.