Nope, those manufacturing jobs were lost regularly and consistently throughout his Presidency.
No, I don't think steel mills magically run themselves but the industry has a strong track record of higher productivity - it having increased 25 fold over the last 30 years.
In 6 years time, if that trend continues, the US will be able to produce twice as much steel with the same number of workers. Increased demand will easily be accommodated without having to increase the number of workers.
Now if you're going to talk about jobs in support industries then you'll also need to consider the number of jobs lost among those who are involved with the import of steel.
No, I think this because I'm reasonably knowledgeable about how business works - that being the way I've made a living for the past 30 years. The trend in the steel industry is ever increasing productivity with more steel being produced by fewer workers so an increase in demand for US steel won't necessarily result in an increased number of steelworking jobs.
OTOH, a switch from cheap imported steel to more expensive US-produced steel will result in a significant increase in the cost of steel to those who use it. Those additional costs will be felt in terms of smaller margins for steel users and/or increased costs for the public at large. In an elastic market higher costs will result in lower demand and so the end result will be lost jobs in those industries that use steel (rather like increases in lumber costs will result in construction industry layoffs) - unfortunately there'll be no increase in steel jobs to compensate and once again Trump's gift to the little guy is a place on the unemployment line.