Dear Siegfried!
Once you click the link to the website, there is another link over to the "Suddhadib (Hardeep) Attack RESEARCH" page that lists a lot of backgrounds of the attack.
But to answer your question here in the forum, here is why a Thai ship was target of British RAF bombers.
• Japan was enemy of the Allies
• Great Britain was stationed in India (amongst others, a lot of airfields in West Bengal around the area of Calcutta, like the airfield Digri featured in the attack visuals)
• as in many conflicts, Thailand tried to stay 'neutral' but was threatened by Japan to be invaded
• instead, Thailand decided to capitulate and serve as an ally of Japan (not as an occupied nation but under Thai military flags - however under orders of Japan)
• one of the war efforts of Japan was the construction of the "Death Railway" - a railway line from south East Asia all the way to India to forward war supplies and troops in front of the doors of India
• Britain was forced to bring this construction to a halt (a famous incident was the attack on the bridge over the River Kwai)
• The Thai ship "S.S. Suddhadib" had originally been a regular passenger and freight ship for over 20 years before the war
• However, during war times most commercial ships were subscribed by the Thai Navy to help the military goals
• on the day of the attack the ship had loaded a large shipment of fuel drums, coming from Singapore and destined to be delivered to the construction of the Death Railway so she became a direct target
• next to the S.S. Suddhadib, at least two other major ships were attacked, the HTMS Angthong (main target) and the HTMS Tachin, plus another smaller fuel tanker. All targets were hit that day.