No one is claiming that it wouldn’t reduce his reach, just that a “small group of Silicon Valley engineers” cannot unilaterally “control and censor” his communication. He would still be perfectly able to have his message heard by literally any citizen who wanted to hear it — in fact, he is the US least susceptible to censorship.
But this is a very dangerous argument, because those same tech companies also control web browsers, operating systems and even DNS hierarchies.
You're assuming that the convention of neutrality in these programs would be upheld, but it's just a convention and iOS already violated it a long time ago in the mobile space. It was after all, once a convention that tech platforms didn't attempt to decide the correctness of what people said, on the grounds that they were just platforms and not publishers.
Additionally the issue here isn't specific to some vague category of tech companies. This is happening because of activist employees like those at Facebook. They can and do crop up anywhere, they can and do impose very nasty demands. It's not merely about de-platforming their opponents, but also forcibly platforming their allies.
For example, Kickstarter's employees unionised in order to force Kickstarter to keep supporting a fundraising project that was openly advocating violence against conservatives. This broke all the rules of the site, which is why it got taken down, but such details didn't matter to Kickstarter's activist workforce - who via unionisation were successful in their goal of converting Kickstarter into a fundraising platform for leftist violence.
There's a clear pattern in which hard left employees seek to both erase their enemies and also make their allies immune to the usual rules. This behaviour is tribalistic and destructive. Judging from history it ends in total cultural cold war, and possibly even hot war. At some point people have to say enough is enough, and that not every company has to get into every fight because a few employees walked out.
I guess tech executives are hamstrung by Californian law and can't simply fire employees who don't turn up to work. This is just one more factor making Silicon Valley unattractive as a place to set up new businesses: set up shop in California and before you know it, you'll have ended up with some hyper-aggressive leftist employees trying to take over your company.
The claim I'm responding to asks if the President would have trouble getting a message out, and obviously the answer is yes for some types of messages.