Yes this is true. The processes are tightly link and often done at the same time. Just before section 6.4.1 in the PMBOK there is a short paragraph on this. One way to look at this is hourly rates of internal resources or a vendor which is often static between initiatives. You can plug these number into your PM tool or spreadsheet before sequencing, scheduling and assigning the tasks. Then once your schedule is done, you also have the estimate for your internal/external labor.
According to PMBOK to get activity cost estimate you use the project schedule as input , and to get that schedule you have to input activity resource requirements which is an output from estimate activity resources which use activity cost estimates as an input!!!!!!?