You can fit it just with seatbelt, if you're using it in high back booster mode. The clip shown in the video is for when you're using the seat with the 5 point harness.
Which way do you want to use it? What's the age/height/weight of the child?
The manual I can see for the original Maxi Cosi Titan has isofix + top tether fitting for Group 1 (9-18kg/5 point harness) mode or "Isogo" for fitting with seatbelt in Group 2/3 (15-36kg/high back booster) mode with optional isofix.
Generally when car seat companies have isofix used for stability but the actual fitting of the car seat is done via the seatbelt, they aren't allowed to use the term "isofix" if the seat is not being secured only by the isofix. That's why they make up terms like "Iso-go" or "isosafe" or "isoclick"
Isobelt seems to be their term for a combination of isofix + seatbelt where the seatbelt does the job of the top tether. I think it may possibly be a feature they have for the South East Asia version of the seat, because when you google Maxi Cosi Titan Isobelt, it comes up with that video 4x in different Asian languages and then (for some reason) the UK Maxi Cosi Youtube page. I don't know if that was a mistake perhaps, because I can't find a UK version which has that feature, or any websites referring to that feature at all apart from those videos and a couple of SEA based shops. Or perhaps it was sold briefly in 2018 when the video went up but is no longer available. In any case if there is no lock off for your seat then you can't use that fitting method, it wouldn't be safe. But if you want to use it in high back booster mode, then it doesn't matter because you don't use that anyway.
It is likely intended for places where top tether anchors might be less likely to be available.
The manual is confusing as Maxi Cosi manuals usually are but this is from the FAQs on their website:
For group 2/3 use (from the age of 4 to 12 with the car seatbelt), the ISOFIX anchorages are optional. We recommend that the ISOFIX installation method is used, as this makes the seat more stable. For example, when driving on a roundabout, the seat and the child will stay in place and not move.
https://www.maxi-cosi.ie/car-seats/titan