Using splits.io links set to ingame time (link shows up as https://splits.io/4900?timing=game) means nothing shows up under your run. This is pretty dumb as it's.... the same thing, but ingame time is used for most of the games I run and I'd like to be able to use splits.io

Are you putting "4900?timing=game" in the splits.io field? If so, that would be why nothing shows up at all. You're just supposed to put the ID (in this case just "4900"), not any sort of link. Regarding in-game time, unfortunately the site doesn't currently support in-game time splits (it's using an older version of the splits.io API that didn't have anything for that).

Is there any chance that might be implemented in the future?

I gathered that the reason my splits weren't showing was because the link wasn't in the format it expected but again, most games i play (racing games, often on consoles with less than fullspeed gameplay) use IGT so my realtime is much much higher than my IGT.

I imagine they will eventually do that, though there's currently a lot of other high-priority tasks in the pipe (and there's generally just a lot going on right now) so it may be a while before it happens.