Can non-tubeless-ready MTB tyres be used to go tubeless?
In general:
It is beter to have tubeless ready (TLR) tyres, but the large volume MTB tyres often work well even when not explicitly TLR. It is much less sensitive than road tyres. However, it should not be a supercheap thin model, that obviously would not hold the air. Also, wired models are less likely to seal well.

To your specific bike:
Decathlon does sell some of the bikes with tubeless ready tyres (e.g., the gravel Triban). And it is the case with your bicycle too. If the manufacturer says it is TLR, there is no reason not to believe them. It is also confirmed at the Decathlon page for the tyre https://www.decathlon.co.uk/taipan-kolos-mountain-bike-tyre-275-x-28-id_8511989.html
Most better quality tyres for MTB are TLR, because it is a must these days. MTB really needs tubeless for good performance.