I think there might be some confusion about the pro-rata rules. ILikeDividends was talking about the pro-rata rules on a backdoor Roth strategy, not whether there is after-tax money in the 401k. That is a completely separate matter, and since you don't have any after-tax money in the 401k you don't need to worry about it.
If you want to do a backdoor Roth strategy, which is what ILikeDividend's linked article is describing, then you want to be aware of any current pre-tax IRAs. If you have an existing pre-tax IRA and you try to use the backdoor strategy then the IRS will look at all of your pre-tax IRA's and apply the pro-rata rules across the conversion.
Example1: No existing IRAs. Put 6k in a pre-tax IRA without taking the deduction and then convert it to a Roth. You avoid the Roth income limits and there are no tax implications. This is nice and clean, and the ideal scenario when using the backdoor Roth strategy.
Example2: You have 94,000 in a Traditional IRA. Now you put 6k in a pre-tax IRA without taking the deduction and convert to a Roth. You avoid the Roth income limits BUT the IRS will apply the pro-rata rules including your 94,000 Traditional IRA. From the IRS's point of view you converted 6k of a 100k IRA, and 94% of that IRA was pre-tax. Now you will owe taxes on $5,640 of additional income. This is what ILikeDividends was warning you about. If that $5,640 is taxed at a high tax bracket, which is the likely scenario if you are over the Roth income limits, then this was likely a terrible decision.
Very important note: your 401k is NOT included in the pro-rata calculation for the backdoor Roth strategy. You could have $1,000,000 in your 401k, $0 in Pre-tax IRAs, and complete Roth conversions without having to worry about the pro-rata rules. In other words, if you plan on using the backdoor Roth strategy you probably don't want to roll your 401k into a Traditional IRA because as soon as you do that you subject yourself to the pro-rata rules. This is what ILikeDividends was warning you about when they said, "(with a caveat)*"