Wemo has had HomeKit for many years. The bridge just covered one of the gaps in their support for really old devices, it was not the first foray. I have a WeMo from the middle ages that needed the bridge, and another WeMo from a couple years ago that had it from manufacturing time.
An app cannot provide an “API” in any real way. We use WeMo’s actual network API. If they changed it (totally unclear from their marketing fluff), it presumably won’t work anymore in which case you use HomeKit. Or just don’t update your firmware. The stuff that doesn’t have HomeKit is so old that it’s probably better to stop updating their firmware anyway (or replace them).