Not true. There was some suspicion on the part of both Dönitz and his men (which is why they added a code wheel) but none of the U-boat memoirs published before ultra was declassified mention concerns about enigma except in passing.

In contrast they attributed getting attacked after sending in a position report to radio triangulation equipment allies had, called huff-duff.

And in most cases huff-duff was the reason they were attacked. Bletchley Park was too slow to provide an actionable attack vector off a position report. Instead ultra was used to route convoys around the u-boats. They experienced ultra as empty ocean they they hoped they would find a convoy.

The one exception was the "milk cows". These were resupply subs that were to rendezvous with u-boats in the open ocean. Dönitz would send orders for a rendezvous, bletchley would decrypt and send orders to a "hunter killer" group consisting of an aircraft carrier and destroyers to attract the two subs while resupplying.