That is partially true due to the design of different buildings. 85% of the stadium will be using std steel sizes and lengths that's how they keep costs down in the design process.
However bespoke buildings can be 18 months from order to supply you are right. No fabrication/production company will stop producing standard size material continuously day after day to make a one off order like this.
Unless you own the place of course.
There would still be the issue of steel quotas for imported steel though.
From Reuters.
LONDON, June 30 (Reuters) - Britain said on Wednesday it would introduce new regulations to help defend its steel industry after accepting a recommendation from its newly established Trade Remedies Authority to scrap some quotas on imports.
Trade minister Liz Truss said the government only had the option of accepting or rejecting the recommendations in full, which said quotas and tariffs should be retained for 10 categories of steel for three years, and revoked in nine others.
However the minister said the government would now introduce a public notice to launch a temporary extension on a further five of the 19 steel products for one year. "Imports outside the quotas will face a tariff of 25%," she said.
The European Union introduced quotas in 2019 when Britain was still in the bloc to guard against steel shipments normally destined for the United States being redirected to Europe after U.S. President Donald Trump imposed 25% tariffs.
The UK steel industry, which generates about $2 billion in turnover each year, had warned the government not to remove any of the EU's so-called safeguard measures, saying it could spur a flood of imports.
Truss said the extension would give the industry time to appeal the TRA decision, which was made before Brussels said it would extend its system of quotas and tariffs for a further three years.
She added that the UK would also review the Trade Remedies framework, a post-Brexit UK-specific system which is designed to protect sectors against unfair trading practices.
"The Trade Remedies framework was first introduced in 2018 under the previous government," she said. "The current government will review it to ensure it is up-to-date, champions WTO rules and is fit for purpose in the post-COVID world."