"Well I finally noticed this sentence that answered my initial question:

"Only individuals owing capital gains tax are required to file a capital gains tax return..."

You mean how they tell you a business would not be responsible, and this is for individual taxpayer and not business...

"This tax only applies to individuals. However, individuals can be liable for the tax because of their ownership interest in a pass-through or disregarded entity that sells or exchanges long-term capital assets. The tax only applies to gains allocated to Washington state."