That's because they are tolerant towards personal choices, not political ones. There's a big difference, and sticking them in the same bucket so you can shout 'hypocrisy' doesn't constitute a useful analysis.
There are plenty of voices of all different political persuasions here on HN (and some in each group feels they're in the persecuted minority). I don't think your parent is suggesting that they need to connect their HN account with their true identity. It is useful, however, to maintain a relatively consistent, if pseudonymous, account as that benefits the HN community as a whole.
I cannot count the number of times I hear supposedly tolerant people yell "Freedom of speech doesn't mean freedom from consequences." Where the consequences is usually violence. But thats OK, because it is not the government doing the violence.
Being coy about being a Trump supporter is a necessary precaution.