[Quoth]

Yes it is. They basically only have EPUB3 for fixed layout (which is garbage on phones and eInk, usually needs 10" tablets) and Multimedia on iOS and Android.

So for multimedia, a dedicated app is better for iOS and Android.
IMO Fixed Layout and/or Multimedia (Video, Gifs, image maps, audio) are not real eBooks. The epub3 spec should never have been called epub, but something else.

Kobo supports a subset of elements from the EPUB 3 spec.

Multimedia: No SMIL on eInk, Mac, Windows etc. Only Android and iOS Apps.

SMIL (Synchronized Multimedia Integration Language) is supported for FXL titles on Android and iOS. Audio files must be encoded using Apple iTunes AAC-LC mp4-v2 codec, 256 kbps.

Note: The Kobo Android platform does not currently support:

Non-Linear playback
Text highlighting for SVG text
Using decimals when the time is already specified in milliseconds. (ex. clipBegin="47808.823ms" clipEnd="48330.373ms). This may cause the audio to stop playing or to not start playing at all.
Using more than 4 decimal places. (ex. clipBegin="4.82333" clipEnd="7.45334ms). This may prevent the audio track from playing at all.

No multimedia content in Comic Book style fixed layouts:

The Kobo Android and iOS platforms will render FXL EPUBs that meet certain criteria with an image-based Fixed Layout reader. This reader features significantly faster panning, zooming, and page-turns than the standard one. Designed to enhance the reading experience of comics, it works for any FXL EPUBs composed entirely of images.

Basically if it's got multimedia, it's not really an ebook, only iOS and Android supported. So a Custom App would be a better experience and cause less confusion.

Multimedia Support / Media Overlays
Testing across platforms for EPUBs with multimedia and other media overlays is recommended.
Embedded audio and video is currently supported on Kobo’s iOS and Android platforms for all content.
Windows does not currently support embedded audio and video. Kobo eInk devices and the desktop app do not support embedded audio and video either but will display any content included as a fallback using the switch element display instead. NOTE: This functionality has been deprecated in EPUB3.2.
Autoplay Functionality is Not Currently Supported

Marketing fixed layout "Kindle" ebooks and any ePub3 that uses fixed layout and/or multimedia is just confusing the users, the market.
Such content is better done as dedicated comic formats, PDFs or dedicated apps.
No real mainstream ereaders implement all of epub3.
Fixed layout and/or Multimedian are not real ebooks, they are print simulations or interactive documents.

Multimedia is only on iOS & Android

Audio Video Issues on Android and iOS Platforms

Kobo often receives content with embedded audio that does not work on one or both of our iOS and Android platforms (the criteria for activating such content is that is pass QA on both). This is usually either because the content has not been designed and tested for more than one reading platform or because the media, codecs, HTML or mimetypes are not compatible with one or both of the Android and iOS devices at all (i.e. even outside of the Kobo app).