Yeah, they probably meant just a regular rollover 401k. I have both a rollover and a Roth, because one is from previous employment and the other is what I contribute to biweekly

yes Roth is not free you pay taxes when you convert as income vs paying taxes later after you retire for traditional 401k typically most people take far less income in retirement so you tend to be in a lower bracket than your earning years so deferring tax makes sense but nobody can tell you for certain how much taxes will be in the future or how productive the markets will be so ultimately it's all a guessing game

If I'm remembering correctly IRA is pre-tax money and rothIRA is post-tax money, and you can do a Roth conversion on a traditional IRA if desired, pay the income taxes now then have no income tax to pay on it when you pull out at retirement and it's gained quite a bit through compound interest

Edit to add: I mentioned rolling 401ks into a rothIRA or IRA because I see it as incredibly risky to leave your retirement in a 401k account controlled by a previous employer. Employers get to choose what financial institution holds the 401k funds, as well as manages employee and former employee access, so they could choose to cut costs and transfer the 401k funds to an institution with higher account fees for you for example. Also many 401ks when you depart an employer get automatically converted into IRAs and may be converted into an IRA populated purely with low return CDs/bonds and high fees (one of mine did exactly that actually!) so rolling it into a (roth)IRA with a financial institution you trust and can control the trajectory of those funds is both safer and financially smarter! I've even heard of folks having their entire 401k drained by fees from the IRA it got automatically rolled into when they left the organization