What fire effects have these previous wooden coasters specifically had? Could it be as I've suggested with my Valhalla thinking that Wicker Man does indeed push the fire effects further than previous coasters, thus meaning Merlin can just about get away with their 'world's first' claim. It may be more impressive than previous fiery coasters perhaps?
Yes, I do think this is what they're trying to get at with the world's first line, as obviously this isn't the first wooden rollercoaster to use fire in some form, and I imagine they're fully aware of that - they did go and ride Wodan for product research. In Wicker Man's case, fire (whether real or not) is clearly integral to the whole ride experience rather than a minor feature.
Really Merlin's world first claims are just their way of summarising their attractions' USPs in a concise and marketing-friendly sort of way - remove the words "world's first" from their claims and you have a stripped-back description of whatever's being marketed. That's not to say it isn't a bit of a tenuous world first, which is open to misinterpretation, especially when compared to things like "world's first vertical drop rollercoaster" or "world's first 14 looping rollercoaster" which are arguably pushing the boundaries of the industry in a much clearer way. However, if "world's first rollercoaster experience fusing wood and fire" is the only thing someone hears about the ride, they should have a reasonable idea of what the gist of Wicker Man is.
Ultimately, these sort of marketing lines aren't going to be fact-checked, and other parks aren't going to take legal action over their subjective interpretations.