I'll take your word for it that in practice it hasn't fleshed out that way. Theoretically he's right though.
If a seller (like China) has a 40% profit margin (just go with it), then can certainly cut pricing on their side to offset the tariff. It could also spur competition to more friendly trade partners.
Theoretically, in the short term it works similar to an excise tax and the respective elasticities of supply and demand. In an excise tax the charge is on the producer or supplier. In a tariff the tax is the demand side, the buyer. The "tax incidence", the degree to which each share the "tax incidence" and loss is dependent on the elasticities of each side.
And, in any event, there is also always what is called a deadweight loss, the lost economic benefits to both sides from reduced production and consumption aside from tax loss - meaning that the economic benefits of a full transaction is lost to both parties.
In your example, there is a 30 percent tariff on the gross price (then added to the current average of 21 percent) which will significantly reduce demand fro US business and consumers. Suppliers will then compare the difference between a much-reduced profit margin by lowering prices, and some will decide it either means going out of business or routing investments and production towards other industries and/or other customers with better margins. They may shift production to items that are more inelastic in American markets, things that American businesses and consumers must have and will pay for in spite of tariffs (eg rare earths or lithium batteries) as opposed to decorative or other items.
This isn't a matter of what they can do, it's a matter of what markets will do whenever prices or costs are mandated at other than market equilibrium prices.
The consumer in this case might certainly pay more for a similar product but if it is from a friendly instead of an adversary, it might have worth there .
I really wish that we would have started the balancing process ages ago (to keep some of the manufacturing here in the US) but we have no one to blame but ourselves for wanting things as cheap as possible.
Business is business. American value-added manufacturers large and small depend on supply chains where they are reliable, deliver sufficient quality, and do so at at the lowest cost that to secure profits (or stay in business) and they also measure it against opportunity cost. When they pay the 30 percent tax they do what the Chinese supply side does, start looking for products, customers, and suppliers who provide a better deal. Again, they will look for demand inelasticity, meaning change their business or customers to something people desperately need and who will tolerate price increases.
Again, some will go out of business, some will change to some other business, and some will cater to clients who are able and willing to still purchase at much higher prices. (which is how Toyota responded to US sale quotas in the 70s and 80s by switching to luxury cars, eroding US Auto makers dominance).
This issue is rather dated, one that was popular in the 1970s and 1980s and pretty much accepted as natural till Trump decided to pull it from the graveyard. Frankly, it didn't work when the US had a much stronger industrial base so there is no reason to believe that it can undo after the fact - or that he should undo it.
We are ahead because we are the world's reserve currency, it has been since WW2 our "gold", something of inherit value as a trustworthy currency managed by a trustworthy country. Anything that undermines that, from our increased deficit spending to Trumps attempt to disinter mercantilism is going the threaten that. Not being the adult in the room will have its consequences, and blowing up the international trade system will cost us dearly.
Trump has his economic priorities all wrong, first and foremost is finding a way to stop increasing federal borrowing much faster than our GDP growth. If not, none of this will matter.
